Originally Posted by Ngchen

Note: the following is a CONJECTURE, and I cannot say that it is correct. It is an educated guess only.
Here's a conjecture as to why MR bulbs last so much longer than flashlight-style HPR halogens. The HPRs have just one glass shell covering the filament. If it leaks due to age or whatever reason, say thermal cycling, the bulb is toast. MR style bulbs, OTOH, have a second cover that I am guessing is airtight too. So it would take TWO leaks before oxygen would enter the chamber with the filament and kill the bulb. This conjecture is supported at least by a comparison of rough lifetimes. HPR - 50 hours. 50 squared (assuming equal seal quality) = 2500, and incidentially, MR bulbs often have lifetimes around that value.

If leaks are the main killer of halogen bulbs, then this conjecture would make sense.