When people overlook exploding blood magic bomb explosions and herculean feats and the presence of freaking elves and alien looking things (Qunari) and then fixate on the mere fact that this relatively hot looking character outfit is somehow the only impractical thing in this wild and mixed up universe, then call me crazy but that starts to look like some kind of irrational fear of sexuality rather than logical analysis.
Qunari don't exist (well as far as we know) at all so therefore practicality judges they shouldn't exist in game. Women shouldn't even be able to use two hand swords, and who knows what else.
It's about making a believable world. You set up the rules of the world (which you invent), and then you create the rest according to such rules. Then you have a world 100% invented by you and still believable. If you for example invent a planet that orbits around two suns, then you should account for that. Can't have just pretty landscapes and everything else is just like on earth. This is why there is no North, South, East or West in Discworld.
So what explanation could we possibly give to have Cassandra go to war in a bikini? Remember she's not there to have sex (so sexuality has little to do with the outfit), but to kill her enemies.
Also, I don't think it's fair to say people only point out this as the only impractical thing. For example, the issue of Hawke wearing robes and a staff while trying to hide from the templars and how jarring it was. It's another example of the game not playing by its own rules. So it's not the only thing. We're talking about this in this thread and we'll talk about other cases in other threads.