I never really thought about it until recently when I noticed a lot of the NPC Miqo'te rolling "R"s. And I figured, why the heck not. Assuming that one needs to vibrate one's "airpipes" to create a purr, I did consider the fact of changing the way speech sounds due to it. But if you do not try to purr when speaking, I should think that it doesn't affect speech much. I notice IRL cats can't "speak" and purr at the same time, and they cannot eat and purr at the same time (I've had several that will try), so if they do not speak, I would think thta the ability for purr wuold not affect speech to much. Anmd on the thing with having cat-like tongues, they would have no use for them. Could you imagine a Miqo'te trying to lick them selves clean in an effort to groom? I certainly can't (but I had fun and a good laugh trying to imagine it).