Most of you guys sez that it looks ugly... righto.... uh.. ok, your opinion.
I need to know why...
...people don't finish their subject lines.
Another reason could be so it's easier to distinguish between the rings you drop when hit, and the rings that have yet to be collected.
There's also another (more far-fetched) situation in which you want to know whether rings are being affected by gravity or not, think about the rings that pursuit you when you have an Attraction Shield and then you lose that shield but don't get to collect the attracted rings. In that case, those rings bounce until they stop still on the floor and are affected by gravity, while floating ones are not.
Bear in mind that this is just my opinion, you may not have any reason to want to differentiate both kinds of rings, but as Fawfulfan said, it's best to (not always though) follow a convention because that way you're bound to make your maps appeal to a broader audience.
Think about it. Does it say anywhere that we have to make rings float? Does it? I say do whatever you want. If you really want your rings to be like that then do what you want, but I'm just saying that it is not required that we need to make them float.
The rings are easier to collect when they're floating. You can run into them, or short-hop into them, whereas the latter is not possible when the ring is half-buried in the ground. The base concept of Sonic Hedgehog is the ability to play without breaking momentum, creating a sense of speed; this is why all of Sonic's attacks can be done in stride without slowing yourself down, and floating rings gives you more freedom to do this without sacrificing collection.