The "group or not" situation was partly what faced me. But I also just wanted to open up the performance (holiday concert at a state botanical garden) to more members of the community band I was in. I would have preferred a larger brass choir, but it turned out that a bunch of us already had the Tuba Christmas books, so ... etc. The euph players ended up bailing on me late in the game anyway and so we had something like two tubas and three trombones.

For occasions like that, it's a perfectly okay thing to do. And the audience will greatly appreciate it.