I agree with fenderman on the advantages of varifocal, but here are my thoughts on extra-pixels and the cost-factor,
I'd like a varifocal feature on every cam I have, but it boils down to overall cost. If I'm buying two varifocal turrets, that's about $140 more I'm spending for two cameras, when I can be buying three for the same price through a fixed-lens (you'll be satisfied provided you do your homework and plan it right). Though the IPVM tool helps give a rough idea, it doesn't work precisely well due to the possible artifacts in the imagery. Having at least one varifocal is essential because it can be used as a test-bed in order to determine whether we need a 2.8mm, 3.6mm, or a 6mm lens. A few extra pixels may not be bad to have as it can potentially give better details in terms of which direction the object was originating from and heading towards, etc. Everyone should be having a moderate setup, not too expensive, not too cheap, and well-thought through.
they are, how much is your time worth? a good lens will cost you as well...also note that a with a varifocal you can play in between the fixed lens sizes to get the EXACT field of view you need so you dont waste pixels..