Sorry, I didn't mean to imply that it was ILM's decision to scrap the D. But I seem to recall hearing them say that it wasn't an easy model to work with. Of course, they did some of the original model work for "Encounter at Farpoint," but I don't believe they were the ones who built the model and they didn't work on TNG after the pilot.
I just get the feeling that different effects houses have their own unique ways of making motion control models and that ILM didn't care for the way the A or the D were designed.
Actually the 6 ft model WAS built by ILM.
It wasn't designed by them and I guess they weren't too thrilled with the colors and the profile (who was? remember Doohan's comments about it?), but shooting it was not a problem, except for the fact that so much detail was basically applied with chalk and would come off (which was more of a problem over time, and why ILM did a pretty heavy-duty repaint for GEN.)
Make no mistake, though, they really did demolish the paint job on the refit for TWOK. When you first start seeing ILM shots (rather than stock reuse of Trumbull TMP shots) in TWOK, it is when RELIANT is sneaking up on them, and the ship looks like going to warp knocked all the paint off while scratching the surface something fierce.
In one of those shots where ENT is foreground right while RELIANT is bg left, the paint job really looks like an Estes Rocket I had that I launched with an "E" engine (super-powerful.) It went up glossy white, and came down a LONG time later and that sucker was practically grey. Reinforcements stripped off the fins, and the only reason I got it back at all was the chute didn't come out, otherwise I think it would have drifted for miles. I think it was a replica of the NIKE-X for those who would care about such things.)