Interesting factoid about the use of the Warbird to chase the scout in the Defector.
That episode boasts some new, smaller and more detailed models than had been used before, and this was the first episode to feature them. The warbird, the scout, and even the D were all new models, and, in the case of the Enterprise and the Warbird, they were smaller and far more detailed then before, so using the Warbird was a conscious decision, and it made a great opening for this show. There is even a shot which all three vessels are seen at once.
Are you sure about that re the Warbird? The Warbird model was never very big, and it seems unlikely they'd have gone to the trouble of building a new one for as seldom as it was used
From Memory Alpha on this episode:
This episode featured three new studio models to the series, including the new four-foot miniature of the Enterprise-D, the Romulan scout ship and the second D'deridex-class model. All three can be seen together during the stand-off scene during the first act.
There is also a screenshot next to this paragraph.
The thing is, Romulans are major adversaries in Trek, and the first Warbird was built in the first season, and was not too detailed. It looked particularly dated already in "Where Silence has Lease" so the producers - who, if nothing else, want the show to look good, decided that building a model of it that at least lived up to the quality of the Enterprise was appropriate. Let's not forget the climax of the show, in which multiple Warbirds were necessary (they used the same one, obviously, but in order for it to look good, they had to build a new one.)
Sure it's expensive to build a model. It's also expensive to hire a guest star, contract a director, or build one-off sets. They do all of these things when they make a show.