The only reason I would agree with disabling is the achievement Pop-up. Can't wait for that pop-up randomly getting into my teamfight, screwing up at least on of my clicks, obscurring my view from potential enemies, or anything like this. As long as you can disable that i am ok with that. Otherwise it would feel like fighting advertisment in a MOBA.
Couple things on this:
- Milestones are mostly transparent, and health bars appear on top of them. You can also click through them as if they aren't there.
- Milestones will wait to appear until you're out of champion combat. If you're fighting someone, they won't show until you're out of combat (similar timings as Ocean Drake's buff, for example).
- You can mute milestones with the following settings: Show All (including enemies), Show Self + Team, Show None