Its pretty much at that point anyway.
Besides, there is plenty of ways to disguise themselves if they wanted. There is also the the fact that Tatsuya doesn't need much in the way of protection. He won over another war-class magician quite handily.
There is some merit to holding back for Miyuki though. Im pretty sure I already mentioned that in a previous post. That's probably at least 80% of the reason he hasn't done so. I just meant that it wouldn't personally inconvenience him all that much. Seems kinda mediocre for a final spite. Im sure they could pull off something much more sadistic.