Well, for me there were a couple of reasons:
1. Dead batteries suck! And, when you're running a circular saw, saw-zall, or pretty much anything else constantly, dead batteries happen very frequently.
2. Batteries are not cheap, and they don't charge fast. Even with 5 batteries and 3 chargers, a good-sized project + my questionable competence at most things one can do with power tools often leads to waiting for dead batteries to recharge.
3. More power:) You've overclocked your computer, so why not overclock your power tools?
4. I *think* this'll be a bit more energy-efficient than recharging and discharging batteries... certainly more earth-friendly if you take into account impact of metal used in extra batteries you won't have to buy...
As usual, this is not the safest or most professional of projects. Please don't hurt yourself any more than you'd like to:-)
Here's a video: