I'm looking to replace the one on my machine, and I can't decide if I should go with a spouted like I have now, or try a bottomless. I don't really make two espressos at once, so the double spout is not a necessity for me.
Debugging is the primary purpose. If you have a well dialed-in recipe, there's no mess. If it's not, it can make a mess, but then covering that up with a spouted portafilter doesn't change that the taste is not so great. You could get a small mirror to keep an eye at what's happening to your shots from a bird-eye-view post-dialing.
Other than debugging, it looks cool, you can shoot coffee porn videos, it can fit taller glasses, etc.