Except those general note labels can't be copied to other drawings... The text overrides reset. The text overrides also reset if you change the style. And Fields can't be used in C3D labels.
And MLeaders have the handy ability to have multiple leaders, and use Annotative Scales (including the ability to place the MLeader in different locations for different scales). Not sure what you mean by "a number of different scales"... If you mean you are creating styles for 1"=20', and for 1"=30', and for 1"=40', then yeah, you shouldn't have to do that.
And then MTEXT has all kinds of nifty formatting features these days, including left/right/center/decimal tabs, paragraph spacing, and more. So MTEXT is also handy for many things.
We actually use C3D General Notes or C3D General Segment labels (to control label rotation) for some things, MLeaders for some things, and MTEXT for some things. Unfortunately, this is the sort of thing that makes C3D far more complicated than it needs to be, as there is an unnecessarily-complex set of factors that determine which one we use for what.
We have been wishing for a couple of years now that Autodesk would come up with a "Super Label" that combined the best features of all three of these. Or at least, a C3D label that didn't suffer from resetting text overrides, had width capability, and had MLeader's multiple leaders and annotative scale abilities, which would be pretty darn close to the "Super Label". Just add per-viewport visibility and placement, and we'd be set.