On the other hand, there's also the question of whether anyone will ever notice the extra polys; it's wasted effort otherwise.
Till we start getting stenciled shadows and the like again.

It's worth remembering that just cause they can't be seen now doesn't mean that they always will be invisible.
I tend to agree with WMC on this one. No matter how many times we say that poly count is not the most important factor we still get people saying that they have to be cut down. It just doesn't make sense as good LODding will prevent that huge number of polys ever being present at one time even if it was a big problem.
As long as someone isn't spending 50,000 polys on a sentry gun it's not much of a problem. Sentries should have as large a poly budget as fighters do. They are present in similar numbers and the player tends to get much closer to sentries when playing.