I don't know, when the bug was there and we had mixed melee... a lot of the players were not liking it. Or at least quite a few expressed displeasure with it.
I'm not too fond of it either. Not only could I spawn as a lowly engineer fighting a heavy, but where in normal melee sd mode you always at least had a "fair" chance against your opponent... in mixed you don't in my opinion. I want a fair chance in SD mode, some classes are just better at melee then others and I'd rather have everyone being the same class because of that
It was a bug which was fixed. But we're looking into re-introducing the mix mode in the normal random mode. So every once and a while you all play a mixed class.