Marcus is not needed for Ch 1. He's just extremely helpful.
Zealot is not equal to most units when he joins, either.
9-10 Lance, C's w/ Roy and Alan: 10.4 Str, 12.25 Spd----27 Hp, 8.7 Def, 2.25 Res, 39.45 Avo
Let's see, there's the best unit in the game on Ch 7. Here's Zealot:
10 Str, 13 Spd----35 Hp, 11 Def, 7 Res, 31 Avo (+ move, axes, weapon ranks, con)
You get a second Silver Lance in Ch 6 and Zealot is one of only two units who will have A Lances right now (and right now you also happen to have two Silver Lances).
Now let's compare these two on Ch 7's enemies, starting with offense.
Steel Lance Knight: 26 Hp/12 Def
vs Lance w/ Iron: 5 x 2 (3RKO)
vs Zealot w/ Iron Axe/Silver/Hammer: 7 x 2/12 x 2 (2RKO both ways, but Silver Lance's massive damage can make the difference on whether it takes one or two extra units to finish it off)/23 x 2 (ORKO)
Javelin Knight: 28 Hp, 12 Def
vs Lance w/ Javelin: 4 x 2 (4RKO)
vs Zealot w/ Hand Axe: 6 x 2 (3RKO)
Iron Sword Merc: 28 Hp, 7 Def, 11 AS
vs Lance w/ Iron: 11 x 1 (3RKO)
vs Zealot w/ Silver: 18 x 1 (2RKO)
Iron Bow Archer: 30 Hp, 6 Def, 8 AS
vs Lance w/ Iron/Javelin: 11 x 2/10 x 1 (2RKO/3RKO)
vs Zealot w/ Silver/Javelin: 18 x 2/10 x 2 (ORKO/2RKO)
Javelin Cavalier: 31 Hp, 8 Def, 7 AS
vs Lance w/ Iron/Javelin: 9 x 2/8 x 1 (2RKO/4RKO)
vs Zealot w/ Silver/Hand Axe/Halberd: 16 x 2/10 x 2/33 x 1 (ORKO/2RKO/OHKO)
Javelin loldier: 33 Hp, 2 Def
vs Lance w/ Iron: 15 x 2 (2RKO)
vs Zealot w/ Iron Axe: 17 x 2 (ORKO)
Mage: 23 Hp, 4 Def, 8 AS
vs Lance w/ Javelin: 12 x 1 (2RKO)
vs Zealot w/ Javelin: 12 x 2 (ORKO)
And, dun dun dun......
Steel Lance Dragon Knight: 34 Hp, 13 Def
vs Lance w/ Iron/Javelin: 4 x 2/3 x 2 (5RKO/6RKO)
vs Zealot w/ Iron Axe/Hand Axe/Silver: 6 x 2/5 x 2/11 x 2 (3RKO/4RKO/2RKO)
Zealot kills in atleast 1 less round than Lance against all enemies, and in most cases he actually doesn't even need the Silver Lance to do it, that's like icing on the cake (2RKO vs 5RKO on those Dragon Knights).
And I won't bother to give the numbers on defense, it's clear that Zealot is considerably better at that, too.
Now for fun, instead of comparing him to the top ranked unit, let's compare him against an upper mid.
6 Lugh: 6 Mag, 8.5 Spd----18.5 Hp, 3.7 Def, 6.5 Res, 23.8 Avo
Should I rly even bother? It's not that I'm unwilling to give lots of stats and numbers on how much better Zealot is here, it's just that it's unnecessary as long as you're not blind. The Dragon Knights OHKO Lugh, while Zealot is 3HKO'd @ ~31% hit, and takes 10 rounds to die on average. Mercs on this chapter can get 12 Spd, so if Lugh winds up with 8, he is ORKO'd. Meanwhile the same Merc would do 2 damage to Zealot. Lugh's offense vs Knights, with a 12 Def/Res gap, is nearly the same as Zealot's Silver Lance offense against them (actually Zealot is still doing more damage, though).
It's clear that Zealot is definitely valuable when he first appears, and should remain useful throughout the Isles at worst. He can bust out Killer Axes 2 chaps before sword/lance units get their Killers, and assuming that his doubling remains steady, Lance doesn't catch up with his base Hp/Def until after promotion, so he'll obviously still be good for quite a long time.
Lugh is ranked even lower than Lot, whom I used for my Marcus comparison, and not likely to even be used as filler material until Gonzo/Echidna/etc start to join. I hardly think a tier gap in Lugh's favor to be justified.
I don't have enemy stats for Ch 14 yet, but I shouldn't need them, it's blatantly obvious that Cecilia's jointime performance is a joke compared to this. If being a 3rd or 4th string healer for half the game outweighs what Zealot has, then I'm gonna go make a topic arguing L'Arachel to at least the top of lower mid on the FE8 list.