Mechanically there's little difference in function between various cage lengths of the same derailleur, so there's no problem with more cage than necessary, other than weight (the RD + a few extra chain links). So the only argument for shorter cages would seem to be the cool factor issue.

But there are subtle benefits to shorter cages. First, most makers use the same cage spring regardless of cage length. That means a shorter cage will wind the chain slightly tighter, which means a quieter ride, and less chainstay slap (when coasting) on bumpy roads. There's also better ground clearance which can be very helpful off road, and lastly if you often have to pack the bike in a box or car trunk, wheels off, it's more compact with a smaller RD, and the cage unit is a bit tougher agains abuse.

So overall, I feel that it's best to use the shortest cage derailleur that'll serve. Of course, those who might be expanding the gear range in the future might want to provide some cushion, and opt bigger.

I also have another reason for my short cage bias. I'm a minimalist. I don't use anythng more than necessary for the job in general, so the shortest cage possible fits in with my overall temperament.