I ran one fall and winter WITHOUT a fan because my fan clutch was always engaged. I got a new one before spring came around, but I never had one ounce of trouble with high temps. I'm sure you could never get away with that in the summer, especially around here where triple digit temps are the norm, but if I could drive around without a fan at all I'm sure you'd probably be fine with just the electric one.
Now I did always keep the fan and clutch in the back of the truck so i could put it on if the temps were ever to climb so i could would also recommend that you do that as well, just in case. the mechanical fan is really easy to put on, you just thread it back in place and tighten it up hand tight and it will tighten the rest of the way once the engine starts running.
As far as MPG gain, maybe I saw up to 1 MPG, but nothing significant, and that was going from having my fan stuck on all the time to having no fan at all. The possible small MPG gains are totally not worth the risk to your motor if you undercool it. I did notice that I cold rev the motor up quicker without it though, but again, the gains were nothing that was going to turn my truck into a NASCAR truck. I can't remember my exact numbers, but I think it gave me an extra 1/2 second on 0-60 times. Again my fan was not working, it was totally stuck on so you wouldn't see even that much difference if your fan clutch is working at all.