False. The black Knight, Tauroneo, Sothe and Nailah (Untransformed) can all tank while not kill stealing through counter-attacks (Unequip weapon).
Black Knight: Exists for 2 chapters, one of them Edward is not even part of, so in practice he's around for one Chapter. Not really a standard. Also, the chapter you use him in is big and enemies have a lot of free shots; he can only divert so many hits as a result.
Furthermore, you use the EXP Stealing Requirement against Zihark...Black Knight is the EPITOME of EXP Stealing. He gets 0 EXP from his kills, starts off with maxed out Weapon ranks (well...I think he has an S in Lances, but for him, that's effectively maxed due to lack of Wishblade), has his level capped, etc. If Zihark steals EXP, I don't even wanna know what you call Black knight.
Tauroneo: Exists for literally one chapter (2 maps granted); how is he saving you in all the chapters you can't use him in? Oh, right, HE'S NOT. Also, one of the chapters he has involves a 2 Panel Choke Point, so he can only hold half of it.
Worth noting that if you want to rescue as many Green Units as possible in 1-6-2, you're basically sacrificing Jill, Sothe and Volug to the North East, which is 3 of your Tankier PCs, cause you need to send several units up there to divert enemies and kill them, and those 3 are the only ones mobile enough to get there fast enough. This means that you basically have...Tauroneo, Aran and Meg as potential tanks.
Nailah: One Map, she's busy on half of it not dealing with your party, unless you wanna sacrifice Civilians and lose actual Bonus EXP. So she's not really aiding the team that needs help. As for
Oh yeah, Nailah and Tauroneo steal EXP more than Zihark does; they're around less, get less EXP regardless, what have you. Well, Nailah can use GLARE for MOAR EXP, but ahahahah to that being reliable, and the only use Glare has is if you want Nailah to immobilize an enemy without killing it (basically, a filler turn), but its hit rate is so abysmal, its basically a "eh, sure, why not" move at best. Either way, this is why you can't use the "STEALS EXP ARGUMENT!" cause then PCs like Nailah are suddenly diminished in use like this.
That said, you use the "Don't equip weapon!" argument...guess what Zihark can do too! His evade is much better than Edward's as well, especially if you give him a support with Volug or Nolan. Double Earth on a Swordmaster = Win. And Zihark, unlike Tauroneo, is usable almost all of Part 1 after that, so he's got both Short Term and Long Term benefits to being used, where as Tauroneo is pretty much purely a short term character (he misses out on 3-6 even, and his positioning in 3-12 is such that its hard to take full advantage of him.)
Up to 1-5 eh? Up to that point Sothe by himself can tank well enough and the only enemies that'll be a problem (The boss Laguz) Sothe can OHKO. After 1-5 you get all the meaty goodness.
So throw away all your EXP to Sothe, eh? This coming from the guy who whined about EXP Stealing. Oh sure, Sothe can hold choke points with no weapons...but you act like your Ranged Weapons are unlimited. I mean, yeah, Micaiah and Leonardo have plenty, but how many Wind Edges you have? And how accurate are those things? Furthermore, did you forget that 2 PCs want Wind Edges? (Nolan and Aran are the only Hand Axe and Javelin user respectively, so its not as big a deal for them.) These weapons aren't storebought, and they're pricey in Aimee's Shop.
Also, there are MULTIPLE Choke Points to hold; Sothe can only hold one. Sothe is good at what he does, but he can't be in 2 places at once. Furthermore, I think this is still a chapter where losing PCs = Instant Game Over? So you can't just say "x PC is expendable!"
That's the problem with relying on one PC; they're ONE person, they can only handle so much. Even immortal freaks like Black Knight, Laguz Royals, Giffca, etc. still can only divert so many enemies. Weaker PCs are liabilities for this reason.
It'd be one thing if Edward could hold off sections on his own, or alternatively, on his own with nothing more than a healer, then he wouldn't be a liability, but he can't. FE10's early game isn't big on these easy chokepoints to hold barring a few stray maps (1-4 comes to mind), and those choke points often allow you to get attacked from multiple angles anyway. This isn't like FE6's Fir who works away her liability by joining on a map that works to her advantages BEAUTIFULLY (Evasive Swordsmen vs. FE6 Axes, with a map layout she can take full advantage of), and comes with a weapon that mitigates her early game offense issues too, no.
In any event, your entire argument is filled with double standard as I noted earlier. You slam Zihark for stealing EXP, then say Sothe does not, even though you can use the two in the EXACT SAME VAIN? How is that fair?
EDIT: As a side note, think this discussion maybe should be moved to the FE10 Rankings Topic? Shouldn't be filling up this topic with nothing but FE10 talk. Just a thought; we have a topic specifically for this kind of discussion, after all.