EXP Runes

I believe that you can use them in conjunction with Zilean to get level 3 off of two buffs. Gimmicky, but it can be pretty helpful.

But yeah, they're quite bad unless there's a specific exp breakpoint you're trying to rush to.

I could see them being used on supports, especially if you intend to intentionally miss xp so your adc outlevels the opponent's. In general though, I think most avoid "useful if you fall behind" strategies since they figure other options may have given enough of an advantage to not fall behind in the first place (or to get ahead). I personally find other runes just give better benefits across the whole game. my biggest decision is whether to go for flat or scaling runes :p