Time for a large post.
It is easy to call Thief utility an easy positive, for they collect items that help the team and are the most likely at doing so, and don't require leveling up in order to accomplish their job. However, is it as really clear cut as this?
I see Thief utility as much like seizing, where it's a goal, but the main importance is actually getting to the goal.
With that said, for Thieves to truly get full credit for the items they collect, and not just consider said items a part of chapter objectives, they need to be able to collect the items by themselves.
What chapters and what items are they capable of getting on their own?
Chapter 6:
http://www.fireemblemwod.net/fe11/guia/capitulo06.htmMost of the credit goes to enemy thieves that are conveniently placed on the map, which steal items for you to kill and gain items from. Only the first chest is guaranteed to be taken by a Thief. Meanwhile, you have to go through the intersection and encounter Armor Knights, Archers, that freaking Mage, and those freaking Social Knights. Alternatively, you may warp to block the exit, making credit go to the Warp user.
Chest items obtained: 5
Chests taken by Julian/Ricardo without help thus far: 1
Chapter 9:
http://www.fireemblemwod.net/fe11/guia/capitulo09.htmThieves can easily get the two chests in the center of the map, with practically no resistance from enemies whatsoever, but a flyer is most suited for grabbing the Dragon Slayer (which is taken by a Thief, so no Master Key is required), due to the distance and turn count required to get there, as well as the terrain that makes a Thief getting there non-permissible unless the enemy Thief is killed first.
Chest items obtained: 8
Chests taken by Julian/Ricardo without help thus far: 3
Chapter 10
Only two chests, one is taken by an enemy thief. The other, unless Julian can kill the armor by himself, will require help on a map where every unit is already hard pressed.
Chest items obtained: 10
Chest items obtained by Julian/Ricardo without help: 3
Chest items obtained with help: 1
Chapter 12:
http://www.fireemblemwod.net/fe11/guia/capitulo12.htmA chapter full of chests. There are two enemy thives, probably bizzaro Julian & Ricardo, but they're too far away from the chests and your thieves will get to them first. If you count the number of spaces, with no obstruction from Marth's starting position, he can actually land on a chest tile while barely losing efficiently, so we can assume he can grab one chest. The thieves can get chests 1, 2, or 3, whichever ones Marth didn't grab. Their free credit stops there, though, because they can't simply walk up and grab chests 4 & 5, due to the Sniper & General.
Oh, and there's simply no hope of getting the Boots without a Warp. By the time Thieves get there, Marth is ready to hit the throne (similar Mov values, and Thieves are lagged getting other chests), so a healer will get credit for the Boots, if you don't hurting warp skip abilities because of it.
Chest items obtained: 16
Chest items obtained by Julian/Ricardo without help: 5
Chest items obtained that Marth can get: 1
Chest items obtained by with help: 4
Chapter 14:
http://www.fireemblemwod.net/fe11/guia/capitulo14.htmGood luck with that Sniper, or it may even be possible to kill the Thieves after they take the items.
Chest items obtained: 18
Chest items obtained by Julian/Ricardo without help: 5
Chest items obtained that Marth can get: 1
Chest items obtained by with help: 6
Chapter 15:
http://www.fireemblemwod.net/fe11/guia/capitulo15.htmYou're better off using a flyer to grab the chests with a Master Key since even Thieves will take awhile to get across the desert, and that's AFTER Gharnef leaves. If not that, warp them there, which Healers get credit.
Chest items obtained: 20
Chest items obtained by Julian/Ricardo without help: 5
Chest items obtained that Marth can get: 1
Chest items obtained by with help: 8
Chapter 17:
http://www.fireemblemwod.net/fe11/guia/capitulo17.htmAnother large chest room, but this time you actually have to get past a bunch of crap in order to reach the chests, a serious credit to everyone else. Not to mention two thieves will grab two of them. Enjoy the Hero and Mages and the Sniper, Thieves aren't likely to be getting there by themselves.
Chest items obtained: 27
Chest items obtained by Julian/Ricardo without help: 5
Chest items obtained that Marth can get: 1
Chest items obtained by with help: 13
Chapter 17x:
http://www.fireemblemwod.net/fe11/guia/capitulo17x.htmI admit, I never actually played Gaiden chapters before, but it looks like all of the chests require help to get, especially since they are so far apart on the map and tend to be guarded by enemies.
Chest items obtained: 32
Chest items obtained by Julian/Ricardo without help: 5
Chest items obtained that Marth can get: 1
Chest items obtained by with help: 18
Chapter 19:
http://www.fireemblemwod.net/fe11/guia/capitulo19.htmNow this sounds like a jackpot for thieves. A bunch of chests with no enemies in them if you know the map layout. Enemy Thieves already get you the mandatory items. The only issue is that, because this map is so short, that you may end up completing it without being able to get every chest, but I'll assume they get them all anyway.
Chest items obtained: 41
Chest items obtained by Julian/Ricardo without help: 13
Chest items obtained that Marth can get: 1
Chest items obtained by with help: 18
Chapter 20x:
http://www.fireemblemwod.net/fe11/guia/capitulo20x.htmI'm not sure if Ymir attacks, so I'll assume these chests are gotten for free. They'll need help getting every other chest, and may not get them all without warp.
Chest items obtained: 47
Chest items obtained by Julian/Ricardo without help: 15
Chest items obtained that Marth can get: 1
Chest items obtained by with help: 22
Chapter 23:
http://www.fireemblemwod.net/fe11/guia/capitulo23.htmTwo free chests.
Chest items obtained: 49
Chest items obtained by Julian/Ricardo without help: 17
Chest items obtained that Marth can get: 1
Chest items obtained by with help: 22
Chapter 24:
http://www.fireemblemwod.net/fe11/guia/capitulo24.htmOne not-free chest.
Chest items obtained: 50
Chest items obtained by Julian/Ricardo without help: 17
Chest items obtained that Marth can get: 1
Chest items obtained by with help: 23
Chapter 24x:
http://www.fireemblemwod.net/fe11/guia/capitulo24x.htmTwo chests, but you have to clear the chapter to get to them.
Chest items obtained: 52
Chest items obtained by Julian/Ricardo without help: 17
Chest items obtained that Marth can get: 1
Chest items obtained by with help: 25
So of all of the chests in the game, only roughly 17 they can get full credit for. For every other chest, either enemy thieves open them for you, or you need the aid of some other unit or need to clear the chapter to some degree in order to make the chests available in the first place, so Thieves are being helped just as much as they are helping everyone else when it comes to Thief Utility.
I think they're fine where they're at right now.