Even a slight drop in voltage causes a big increase in life. For example,
a 5% reduction almost doubles the lamp's life (1.85 times). At a 10%
reduction, the lamp lasts 3.5 times as long.

I guess the question becomes: is there some voltage a bit below normal,
where the halogen cycle doesn't work and lamp life actually degrades?
I'm guessing no.

I'm guessing yes. A friend had halogen lamps on a dimmer, which he dimmed
only slightly -- and they burned out pretty quickly.

The halogen cycle requires a very high temperature. Ergo...