There are actually many reasons for timers, and reasons not to buy non-factory original lamps. Most people aren't aware of these issues, and sometimes they don't manifest themselves as serious issues, but when they do, you better believe people complain.
1. Manufacturer original lamps (Not OEM!) - sometimes have UV coatings on the bulb or the housing. This prevents excess heat from going into the projection system and helps maintain not just lamp life, but projector life. I have heard from several people who have burnt their optics up using non original lamps.
2. Lamp housings warp over time. All that heating and cooling on the metal cage. No, it's not a ton, but over the 2,000+ hours that the lamp is designed to last, the housing is designed to be throwing away with the bulb. Doing a bare-bulb replacement doesn't change the fact that the housing is no longer as 'perfect' as it was the day the lamp was purchased, and me be significantly degraded over that time period. It should be replaced.
3. Lamps can and do explode, but this is a very rare occurrence that may happen at any time really. I would not consider this a prevailing issue with lamp aging.
4. Significant dimming and contrast loss. For those who haven't seen a lamp near the end of its life, you will be appalled at just how dim that lamp does get. The projector loses all of its pop and wow factor, and just looks... dull. This is because the lamp may be 25% as bright as it was when it was new, and projector manufacturers know that your lamp will look terrible after a while, so you need to budget accordingly.
It always seems to be that it's a bit crazy to save $100 or even $200 on a gamble which could cost you a $1,000+ item, when this cost is clearly known when the projector is purchased. I mean, a $250 lamp, which lasts 2,500 hours, costs 10 cents for every hour of use. If you watch a movie every day for a year, that adds up to about $80. That may be less money than it costs to take a family of four out to the movies just once! So, the one-shot of $250 may feel expensive, but so do new tires on the car, or any other piece of preventative maintenance on any product. Suck it up! Your life isn't at risk on this, but a very nice piece of electronics may very well be on the line. If you budget ($80/yr.) then perhaps when the projector lamp does fail, it won't hit as such a shock purchase. Oh yeah! We've got money for that set aside already. Heck! We got an extra year out of the lamp, so overnight that new lamp to our home right away.
