I DO NOT want any in-universe explanations or answers for this. I'm interested, strictly, in the decisions made by the producers.
In The Enterprise Incident we see Romulans using Klingon ships (or Klingon designs for their own ships). It's the first reference we witness of any connection between Romulans and Klingons.
This is explained with one throw-away line, when Spock says, "Intelligence reports Romulans now using Klingon design." Nothing else is made of this in TOS, even though we see more of it in ST:TNG. (In fact, after this episode, real Klingons or Romulans only appear in Day of the Dove and the "alliance" is never referenced.) This, and the need to explain it, seem totally superfluous.
Have any Star Trek producers or staff members ever spoken about this? Why go through this extra effort instead of just using Warbird models for the FX shots?
I can see more off-screen explanations for this than on-screen. It could be that they needed multiple models for the shots they wanted and had more Klingon ship models than Romulan ship models. Or the Romulan ship might have been destroyed, along with other props, when they thought the series was cancelled after season 2. They might have also felt Klingon ships were more recognizable.
Or were they actually planning to use an alliance in later stories that never were produced?