They needed to use the 6-foot for Generations because of the saucer separation, even though it looked more unfinished than the 4-foot model. The 6-foot model had a smooth, unfinished surface, while the 4-foot model had a textured surface that depicted the detail the actual ship would have had.
But that only makes sense for the couple of scenes where the saucer actually separates.
The 6 foot model's smooth surface was not unfinished at all, it was the designer's intent. It's details were incredibly fine and intricate. What you're confusing is the more exaggerated features of the 4 foot model for more detail. On TV screens (in the 1990s) the 6 footer's details don't come across as nicely, so the over-the-top 4 footer translates much more clearly. When you make the move to feature film that's no longer the case.
The Excelsior was easier to work with because it was built to ILM's specifications. Even the Reliant model was easier for ILM to work with than the Enterprise model. The Enterprise model had two problems. It was too big for ILM's motion rig and didn't mate well to it. Plus, it had interior lighting that ILM found difficult to work with. ILM never expected to work with the refit Enterprise model again after Star Trek III; to their chagrin, they had to get it out of storage and clean it up for Star Trek IV.
Yes, this is common knowledge, but it has nothing to do with why the Excelsior model was built. The Excelsior model was built because the script for Star Trek III called for the ship to appear.