I honestly believe that the dialogue, more than anything, is what makes or breaks the believably of a setting. Not all settings need to be believable; if they're fantastical enough or light-hearted enough or over the top enough, you can let belivability slide.
But with a setting like Mass Effect, I think a large part of the draw is belivability, and that's grounded in the characters and the way they talk. They talk like real people in the real world, by and large. And real people in the real world cuss. Real people in the real world don't care if you find their word choice crass, they don't care if they offend you. They don't care if you find it necessary, or immature, or a sign of a lack of education or intelligence. They speak the way they want to speak, and if you don't like it, you can take off.
Of course, in the real world we can tell people what language we do and don't find appropriate to be used in our company and if someone doesn't comply with those boundaries, we can generally choose not to interact with them. The latter ability exists - you get just about everything you need to know about Jack's opinion on your opinion of her cursing after the first conversation, and can cut off contact after that. The former ability is a waste of the game's word budget.
Point is, in real life, people cuss, and we either enjoy it and embrace it, or we avoid it when we can and endure it when we can't. And this is not an aspect of Mass Effect or Dragon Age I want to see removed.
But why should Asari, Krogan, Turians, etc use the same expletives as humans, esp when their mating is quite unique and different? Having Wrex make a ref to a quad is for more immersive to the setting than the profanity uttered by some other races (eg; Aria, some Salarians). After all, the time since first contact does not seem appropriate to alter speech much, Turians as the possible exception. For myself, it often breaks immersion.