It's based on a point that was brought up to me on GameFAQs that I was never able to refute properly simply because the numbers were against me.
Dorcas comes out of LHM at about Level 7. Bartre starts at level 2 on the same chapter. However, if we're distributing EXP properly for the run, Bartre's going to start catching up to Dorcas by the end of 13x and Dorcas will no longer be able to do stuff that Bartre can't, even with Bartre's starting AS of 2. I need to pull up the argument on GameFAQs though but the basis of it was moving Bartre and Dorcas closer to each other.
Dorcas vs. Legault: Combat vs. Utility. Dorcas is good in the earlygame. However, he's the 3rd or 4th best overall combat unit to start with (Marcus, Hector and Oswin depending on how important that 1 Move is) and has defensive problems that the other 3 don't have. Legault is one of two Thieves in the game and is arguably the better one from when he exists to the end of the game. It's hard to judge how important thieving is but the fact that Dorcas can never be the best person at what he does while Legault has that opportunity means that Legault is better in my books. Up for discussion, though.
Dorcas vs. Fiora: Flight is that important. It really is that big.
Dorcas vs. Canas: Here's where it gets tricky. Canas really has one problem and that's his AS. I keep advocating one of the two Body Rings to go to him since it gives him 2 AS (Luna only weighs him down by 1 AS now) and nobody else really uses it as well (you have Isadora who also loves it and maybe Erk but I don't see Erk needing it since his AS should be good enough anyway). Promotion gives him 4 extra AS (3 Spd and 1 Con making Flux cost nothing at this point) while promotion can't dig Dorcas out of his own Spd and Def problems. To me, this is a pretty good example of risk-return. If we give Canas a very uncontested item, we're instantly rewarded with 2 AS. If we give him a pretty coveted item (3rd or 4th Guiding Ring assuming we even want to use them), he gains another 4 AS and immediately saves himself from Spd problems before mention his staff usage (it's not great to start at E staves but it's better than nothing). He's already got a monopoly on Luna which makes him quite awesome vs. some of the later bosses even without a promotion. And Nosferatu can keep him alive when you can't afford to heal him via a staff, along with getting EXP for Combat.
Bartre vs. Jaffar: In all of my runs, Jaffar saw combat in 28x (wasn't much, he wasted a couple of archers and snipers on the treasure platform), 31x and Final. The only one where he's actually doing stuff that makes me appreciate that he was there was Light since he COULD insta-kill a boss. Aside from that, it's really not much. I'd rather use Bartre as a lategame Exp unit when you're hurting for every drop of Exp (EHM broke me badly).