No reason, I use 5.5mm ones by Castle. Every so often you need to spread the males plugs a little to get the "bite" back, but Deans get a little loose too with use and time.

They are easier to solder. Just tin wire and bullet and while heating bullet back up, insert wire and watch for when solder on wire melts. Pull heat and hold for a few secs. Instant connection. I've got 2 pr to solder this afternoon for some new lipos with 12ga wire. Those are big and very hard to solder with deans.

I've read theres an amp level where deans start to get soft, they finally get hot, then bullets are preferred. But it on up there amp wise so I doubt thats a good reason.