Don't expect it to last very. If you don't run it long enough to engage
the halogen cycle, the bulb will burn out faster than a conventional
incandescent.

I'm not convinced about this. I have lots of 12v halogens on dimmers,
and their life doesn't seem any different from those which aren't. If
anything, longer. Nor have they blackened as some say should happen.

It depend on how far you dim them. If you dim just a little bit, they'll be
at the point where the temperature is extremely high, but not high enough
for the halogen cycle to kick in. Result: short life.

But if you dim fairly far down, you'll be below that range, and they'll last
"forever".


And giving an equivalent light output based on some ancient incandescent
bulb no one ever uses?

It provides a reference point most people are familiar with. In 20 years, it
will have disappeared.