Ideally, I think, we'd like to avoid adding too much additional administrative-type items for staff to have to continually maintain. I feel like we'd be better spending more time on moar music & artist promotion than going back and forth with enabling ads. Also, I took it as $25 for global ads on (EDIT: not including unskippables), so waiting for times to get tight (which hopefully they don't) before turning them on seems like it would be a case of too little too late.
But yeah, personally I never had an issue with enabling them on mixes in the first place - especially considering the funds fully go to, as ad revenue always has gone to, maintaining & promoting the site itself.