...what would be the downside?
This is an honest question, by the way - it would mean she could more readily afford to help the jungler when they're clearing Krug/the toad/the dragon, and it might be enough to make her a just-for-fun jungler, but are there any actual problems that this would cause?
I'm wanting to put that banana in the jungle. ;^) I would want it to happen, mainly because AD lifesteal support 
isn't very viable... But it would be very diverse Rito!
(Being a little more serious, I see a problem in the late game where you would throw a star in the jungle late-game to regen your life when the enemy is too far to poke. It would make it hard to counterplay AP or support Soraka in the late game.)
I can't think of any way it would be used for allied junglers besides those few times you will know that the enemy it taking their buffs early on.
Soraka and her ADC trade aggressively with you, you're dodging all her skillshots so they're not getting sustain, things are fine, then she goes into the jungle, heals herself up and is back at the lane for more aggressive trading.
...what would be the downside?
RIOT doesn't want you to play Soraka, or Leona, or Alistar as a viable jungler, and they have admitted it. As always, they used the words "unhealthy" and "toxic" and you know.
RIOT thinks that getting ganked by Alistar is frustrating and he must not be viable as a jungler for the sake of "healthy", but getting ganked by Vi with her 900 range gap-close Q + 800 range targeted unstoppable gap-close R, or having a Lee hoping to a ward to kick you into the enemy turret while dealing insane damage, or getting Zed making a stupid play with his R where he completely messed up but, don't worry! you can spam RRRRRRRRRRR to teleport back, that's just f*cking great, healthy and enjoyable to play against.
If nothing else, they should let it procc on dragon and baron. So that you could actually be of any help in taking them.
