I think it's a question of how gadget oriented you are, and whether you feel you have to have the very best of everything. I have a long standing habit of getting by with the best quality I can buy, without worrying about getting the newest, or most powerful, or coolest, etc. Wasting your money? Only you can decide if its a waste of money for you.

Offroad, as posited by Harlemgirl, the minimum you need is 15 watts? Then what have I been doing for 15 years with far less wattage? Think of all those great rides I would have missed, waiting around until I could afford 15-25 watts? And I never, ever thought "Damn, if only I'd had the 15 watt light, then I could have run this trail." Hell, I've ridden down 6 mile-long canyon access roads with a failing headlamp (Petzl Micro, two double A's) and been fine.

As with cars, if you don't have enough lighting to buy you the reaction time neccessary to avoid the thing in your path, then slow down a little bit. I'm not racing when I'm riding at night, and I'm really not outriding my cone of illumination.