As someone who once bought Kayle on a whim to prove a friend wrong, I believe she may be underrated as a support.
Yes, she does not have the same healing or boosting capabilities as a Soraka or a Lulu, but Kayle is what I would call a battle nurse. She is very capable of juggling between healing a wounded ally, shielding a dying adc, or poking an enemy.
The trick to using her ult is to find the best person to use it on. Early game, I typically use it to save my adc or help a jungler dive. Late game, I pretty much use it on anybody who has the best burst damage, which is immensely satisfying when we get ganked and turn the fight around. During that gank, her E is the poke, Q catches or slows anybody who threatens us.
Her heal is a bit pathetic though, so I don't rely on it heavily, more so to speed up the running away/catching someone part. It is still useable though, and with proper cdr items.
Kayle is one of my favorite champions, and in the right team, she can definitely change the course of the game.
P.S. The champion is only half the support role. The other half is the summoner :)