Is there any reason why i shouldn't have my ECT switched to the Power setting all the time?

In Power mode, it seems like i have to use less throttle overall.
the Normal setting seems to make more slippage

my gas mileage doesn't seem to suffer because of it.

 

No reason at all. The Power setting just changes the shift points on the transmission, the upshifts happen at higher revs. If you're mileage doesn't suffer then use it.