They went from it being impossible to it be possible if unlikely. To me it is a black and white issue, it was not allowed before and now it is allowed. The rarity of the situation matters not at all.
Why not? The setting hasn't changed; there still aren't any Dwarves running around with the Mark of Healing, except with the possible exception of a single PC in each game (approximately 0.000001% of the population of Eberron). Bear in mind that it's easily explained as an Aberrant Mark, which grants the power of a given Dragonmark to someone who shouldn't have it; this still isn't different from 3.5E Eberron.
Have you considered that your reaction to this ultimately minor rules change is just a knee-jerk to something that seems unconscionable on the surface? As for myself, Eberron is far and away my favorite setting, and I didn't like this change at first either. That said, when you keep things in perspective you realize how insanely little this changes anything.
I'm with Crothian on this one; I strongly dislike the change. It's the whole "we can't say no to the players" mentality. I give my players plenty of choice (although as soon as I say that one of them will chime in to disagree), but there are some things that should just be not allowed. The only way an unusual race/dragonmark combination would come to pass is through Rule #1: the DM.
As far as the example of if a player came and asked if he could because it would be a great concept, I'd give him the same response I would if he wanted to play a LE paladin, a fighter with d12 hit dice, or use some odd splat-book varient of the sorcerer. "Nice idea, but sorry, not in this game."
You're aware that "Saying yes to players" is the mentality that designed
Third Edition too, right? (It further informed 4E as well, but to a lesser extent than "Making the game
more fun" did.) I get the impression that the "plenty of choice" you give your players is probably limited to, "Anything in the
Rules as Written, minus the couple bits that I don't like," because your bottom paragraph suggests that you don't like bent rules.
Also, surely you know that Paladins can be of any alignment in 4E, right? I'm pretty certain that each of the "exotic" concepts you described appeared in 3.5E's Unearthed Arcana.