And I'm not saying that a T rating should be sought as a default. THough yes, such games can and in the past have been very good. There are some incredibly popular games, both action and rpg (Starcraft and Planescape: Torment as I've mentioned before)
But yes, also, M rated games can also be great. But being rated M doesn't in itself make them good. And I think it's entirely too easy to go overboard on content simply because it's M rated. Not because it actually adds anything to the game or to the story.
That is something I think developers should be watchful for.
I think that question needs to be framed differently. Being rated M doesn't simply make a good game in the sense that F-bombs, naked people, and dead bodies are all I want from a scene. If that were the case, I would rate all M/R media very highly and that's definitely not true. But in the reverse scenario, there have been scenes which are absolutely brutal and couldn't happen with any other rating which are extremely iconic, the Reservoir Dogs ear-cutting scene being a key example. Oberyn's brutal death in GoT, offering another.
Note that the question was in regards to Bioware's strengths and wanting an M-rating. I don't know how you feel about the DA:O Brood-mother encounter or the DA:I Grey Warden murder/blood rituals, but those are examples of scenes I greatly enjoyed that couldn't happen with a lesser rating.
Even using Planescape:Torment, while that works as an example because it is T-rated, brings up other issues with regards to our rating system. This is a huge part of why I advocate for KotOR 2 as an example deserving an M-rating. It probably offers one of the most despicable moral choices offered in gaming, but because it's not as overt as say F-bombs or naked bodies, it manages to eschew a higher rating. And I can't say there are many moral arguments which could be used to defend as F-bombs/naked people as being more problematic with regard to that content.