The only advantage of having your ryu on the stack it's to propagate sickness, so there's more sick ryus = to more chances of unlocking the userbar
A sick ryu has a skull.
A healed ryu will show you a heart animation when you feed/scooped them. If you happened to be the lucky person to heal a ryugotchi, that one will count for you.
3 healed ryus = new userbar
Check my post the previous page chii, there are some pics shared by others so you maybe it's easier to understand?