I've had 3.0 JB since it came out on my 3g as well. I have no problems with it and I see no point in upgrading to 3.1.2. Everything works fine and dandy for me.
When I was on 3.0 I was experiencing a lot of App crashes. If you're not having those problems then sure you can stay on 3.0, but I don't see the point really.
I don't know bout you, but I can't seem to get new apps installed on my phone. I'm on 3.0 as well and I just bought GTA:Chinawars which does not support 3.0.