Who would you actually take Summoner Specialist on?
I'm thinking that it'd be nice on everyone theoretically because you can get something like ignite everywhere but switch it out later on for other stuff, try to make an early play or something. But I just don't really think that it's worth it for a key stone unless they're gonna buff the 15% CD on summoner spells.
Compared to all other key stones it just doesn't add up for me personally. I just see a lot more consistent power in everything else. Maybe if you had a coordinated team it'd be cool though. Work around teleports, smites, having multiple ignites or exhausts, etc.