3D movement is already in the game and it's not difficult to use
Well, it is difficult to use. It may use the same system as homeworld (I don't really remember how it did it, was a long time ago.), but it's not a very good system.
First, placement in 3d is not shown very well and thus hard to read. (A single perpendicular line from the ship to the plane (ie. grav well) would help a lot). IIRC homeworld had that? (If not, a good example of this system is used for the gravidar in X³.)
The adjustment of ship orientation to the plane destroys every 3d feeling you get (this is the reason why freelancer felt so flat and not really 3d, it adjusted your ship all the time so you were positioned straight. Straight to what?)
3D movement certainly would make things more complicated for the vast majority of people who would play Sins, which is why you're not forced to use it.
Yes, people shouldn't need to use it. But because it's barely included (no structure 3d positioning, ship aligning automatically, no possibility to orient your ships in 3d, hard to read 3d positions, no firing arcs for ships, etc.) it's not even fun to use.
It's very disappointing that some folks feel that Sins is boring or lacking in innovation - obviously we disagree 100% but are continuing to add additional features and tweak the existing ones.
Well, Sins is not a bad game at all, but it has not much innovation, this cannot be disputed. The gravity effect is negligible and besides that, it doesn't have one feature that wasn't in some other rts or 4x game already.
Even the blending of 4x and rts isn't something spectacular, since Sins currently has almost nothing of a 4x game and much more of conventional rts games. Which doesn't make it a bad game. Sins is even a good space rts (well, there aren't much space rts anyway) but nothing out of the ordinary, imho.
In short, Sins is now revolution or rethinking of the genre, but it's still fun to play.