I guess my thought is that, ultimately, if you limit yourself to only what's "needed", you miss out on some tangential stories that often end up being some of the best. Clyde Bruckman's Final Repose again comes to mind. Imagine if The X-Files had been produced in Britain with 8 episodes a series. That episode would never have been made, or even considered.

The recent episode of Fringe centering on the Watcher is in the same category.

That said, there are notable exceptions to my claim. The one that comes immediately to mind is The Girl in the Fireplace, from the first David Tennant series of Doctor Who. Also, Blink, from the following season. (Notably, both Steven Moffat episodes.) That said, Doctor Who shoots 13 episodes a series, which is probably a good compromise between the two extremes.

Also, consider seasons two through four of Babylon 5, which shot 22-ish episodes per season. Which of those episodes were useless filler? Given, a couple, but no more than two a season, and I'm pretty sure they were there intentionally in order to provide some comic relief.