Pete, One of the benefits of writing scores with flexible instrumentation is that one can take either a conventional or unconventional approach to the instruments being used.
Most mid-size jazz ensembles I've heard represent a "little big band" concept -- which is based on sections. The concept that I have for a mid-size ensemble is different. Instead of it being based on sections (or scaled-down sections), I envision the ensemble as being a collection of colors. With that in mind, if one really wants to stretch out and have an unusual collection of colors, the wind instruments could be something like alto flute, soprano sax, English horn, French horn, bassoon, and tuba (as an example). That would be a really interesting sound!
Nevertheless, by using flexible instrumentation, a more conventional horn section could also be used like 3 saxes and 3 brass. In this way, one has LOTS of options.
Another thing that's really cool about flexible instrumentation is how a chart can sound quite different by using different kinds of instruments....like with the two examples I gave above. This enables a director or band to experiment with different instruments.