Hm... I can't say I really buy that. Sounds like a pretty big stretch to try and justify why trans roles don't have to go to trans actors. For me, it's enough to say "There are very few trans roles out there. Why not give them to the talented trans actors (and believe me, they exist) who otherwise are largely ignored or looked over in favor of their cis colleagues?" To me, that's more than enough justification.
I'm really not trying to justify anything, I was just pointing out that fictional societies do not always match up to our society in certain aspects. I believe that any role should go to the most fitting/talented actor. In general, we should strive to hire more minority representatives, but only having trans people in the casting for trans characters seems exclusionary to me. But then, my ideals would only work properly in a more perfect world as well.
One more thing, and I swear I'll try to step back from this thread, hahaha... On the subject of "We should always hire the best/most talented person for the role":
When I comes right down to it, 'talent' and 'skill' are largely subjective qualities, and the entertainment industry is not entirely fair, nor unbiased. I would hazard a bet that if you got two actors, one trans and one cis (or one black and one white) of equal skill, talent, and experience, a casting director might find themselves judging the cis (or white) actor to be "more qualified" or "more talented" without ever realizing that their own biases are impacting their judgement. And that's not to say that hypothetical casting director is willfully transphobic or racist-- it's just that these biases are hard to shake (sometimes even hard to realize we have them) when we grow up in a society that teaches us, through our media, what is 'normal' (and thus acceptable).
And I hate that this is actually an issue, but I know that is the sad truth. I'd also like to thank you for the civil debate. 
Thedas isn't actually a non-sexist utopia. I doubt being trans would be trivial.
Didn't say it was. I said it might treat transsexuals differently from our society.
There are billions of people in the world, to select *the* absolute "best" actor for a role is not possible. It is not realistic to begin with so I don't see why people are actually suggesting it as if it's an attainable goal.
But you can find the absolute best in a specific group of people, in this case, the actors audition for a role. Nobody is saying that we should have a casting for every single person in the world.
So, in the history of film/TV/video games, there has never been a trans actor who has been the most qualified for the job?
That really isn't what Sylvius said. He just brought up population statistics, that while only tangentially relevant to the issue still hold true. Trans people are a minority, that (most likely) means that they are also in a minority representation among most professions, including voice actors.
And it's not as if, as players and not developers, they would know all of the people who auditioned for the role and how well they would have done. There is no difference to the player, just an irrational fear that casting a trans person may mean that a vaguely "better" cis person may have been passed up.
I see this all of the time in discussions of inclusion in games. This worry that by being inclusive, the rest of the game is somehow likely to suffer.
While I can only speak for myself, this really isn't the way I feel about the issue at all. If the developers pick an actor, I have no reasons to think there was anyone better. I just really don't care about who the actor is in his/her personal life, I care about the end result, the character. I will trust that the developers, especially ones who put so much emphasis on their characters as Bioware, will have picked the absolute best actor that was availible.