First understand that while it was the producers desire to get rid of the D (as it was specifically designed for the 4:3 frame ratio that isn't going to be the case for films). It wasn't just about the exterior, it was completely and totally evident that the entire of the ship as built couldn't be legitimately used to look could for theatrical use. You see how much they changed the lighting scheme and changed sets to try and make it more suited for the big screen, it was just a huge issue for them and looking into the future (and I say this as someone who really doesn't like the exterior of the E, interior love though).
As for the use of the Bof Prey its all about budget, everything costs significantly more money when working on film. From set design, and dressing, to actors, to lighting, to effects. It was tough lesson for the writers of TNG as they had really hoped to expand what they could do, then to find out a single phaser fight with Soren, Riker and Worf was more expensive then a full episodes fx on TV.
Remember Trek was after The Motion Picture as a budget film project, that only changed with the current Trek film.
Khan was made for 12 million and was able to get to that degree due to the heavy reuse of sets (thus the same bridge for for both ships), and slight redressing of any new sets, not to mention the huge amount of reused FX shots from whats was initially filmed for the Motion Picture.
Generations isn't the same beast, they had almost no reusable footage that was filmed for theatrical release (save the bird of Prey), they cleaned up a few brief shots from Encounter at Farpoint, but those shots were ever filmed for theatrical release.
The Vorcha like wise was a nice model for TV, but it would never hold up at the theatre.
So for a new ship that means they would have to build a new hero level model, those can run 1/4 a million and higher.
The movie was made on relative low budget, where the bulk of the budget they did have was for casting.
Now they could have gone outside ILM (but at the time, the one time they did they were grossly disappointed by the fx work).
Of course they could have killed stellar Cartography. Though I loved that scene and the FX for the most part. Certainly a thousand times better then the set used in Lessons. OR changed other aspects of the script to remove other FX elements (like crashing into the planet). But to do what they chose to do, they were going to have to trade off with reused shots, redressed sets, ect.