What if... Supports built Mejai?
They're cheaper than ruby sightstones, and definitely cheaper than rabadons, what if sustain based champs like Soraka or Nami built this to gain AP for better healing (both their ADC and themselves gain from this!), and just gained 15 assists? They give supports the scaling AP they need, and they shouldn't die if they're not irresponsible with their mana and health, especially if the support is specifically defense based (Braum, Janna, Soraka, Nami, etc) and not offense based (Karthus, Leona, Annie, etc)
On a competent support who's perfectly capable of keeping both him/herself and his ADC alive, why isn't Mejai viable?

all the time.
but I find that her kit doesn't need damage or extra healing to be good (neither does Janna but the reason I go AP on her is because it's really hard to die on Janna even if she is squishy.)
on
on
is a really underrated buy on
so many times on Janna or ranged supports like Soraka who don't have a whole lot of damage without a real AP build. It really helps in the laning phase and mid game.