If defog gets any sort of usage, it will predominantly be in UU short of one of three things happening
1. OU drops down some better spinners (seriously, UU has the worst rapid spinners of any meta, even worse than NU probably)
2. Every man and their dog proceeds to run mega blastoise (which sadly, will probably happen, causing huge overcentralization of UU's meta)
3. Froslass Remains banned/gets rebanned from the tier along with people wishing to attempt to run other mega evolutions/other bulky waters than blastoise.
To expand a bit, given UU is full of fire types (because of their great offensive capabilities), flying types (because of their defensive/offensive roles) and far too many good spinblockers + hazard setters, I expect defog to make a strong showing in UU. Even moreso if froslass is reallowed, as HO makes life hell when too many layers of spikes go up and you're unable to spin or cough up valuable turns in doing so, or again, because RS is so awful in UU that you'd much rather forgo it if possible. MegaBlastoise will help, but I really do not like the idea of a forced requirement in team building, which is what blastoise will become IMO, due to it then being the only offensive RS in the tier + the current state of hazards in UU's metagame.
Hell, even if OU dropped some spinners our way, we'd still probably overcentralise around megablastoise, simply because we'd be lucky to get something like donphan, who would be overlooked for blastoise.