It's personal preference cosmetically speaking. I just don't like mounting rings at all, all rear routed guitars look better without them, IMO. Kinda reminds me of braces somehow.
Also, you could have a RR guitar with a pickguard and even mount the pickups in it. So you can have a RR with the looks of a FR...
Another thing is that all guitars without direct-mounted PUs that I've played get a lot more feedback. Maybe because there's more air around the pickups? Semi-hollow and
hollow body guitars do that as well (only in a bad way IMO hehehe...).
I've also heard there is a sound difference concerning pickguard material. Dunno how true is that, since I've never tested it.