You don't need much clearance. I think I have about 1mm clearance on my old Colnago, perhaps even less (now 9 speed, 126mm OLD).

The advantage of less clearance is that it is impossible to drop the chain on the outside, and less dishing is required, making a stronger wheel. Perhaps one also needs less lateral range for the derailleur.

Perhaps the only disadvantage I can imagine is that some derailleurs may lack adequate pull to shift back down to high gear. Is there a slight risk of scratching the frame? Minimal? I would think dropping the chain on the outside would be worse.