Chris,
I've given this alot of thought over the last couple years, and have kicked around the same concern. I have come to the conclusion, that at least for what I envision, a xenon Surefire is sufficient for my purposes. Let me explain.
1. I don't intend to keep the light "constant on". The light is used to illuminate and identify a target, not to find my way around things. Think spot flashes until a target is found.
2. I use the P61 lamp assembly for 120 lumens... good for 20 minutes runtime per Surefire. I want to SEE my target out as far as possible, not just know it's a human at 100 yards.
3. I have a spares carrier with an extra bulb assembly and six batteries in it on a lanyard. That will give me approximately 80 minutes of WEAPON light time.
4. For general illumination I use a handheld G2, E2E or E1E depending on circumstances. Again, spares carriers for all of them are stored with them.
5. I'm not LEO or .mil. The most LIKELY scenario is a "bump-in-the-night". If I need more than 20 minutes of weaponslight illumination in that scenario, I'm probably screwed anyway.
Having said all that, in a long-term SHTF / TEOTWAWKI situation, screw the light.... I'll just shoot! No, seriously, that would call for maybe getting an LED light, but that's down the road for my purchases.