The PS3 has also been on top for being the most efficient way of folding, and now more than ever with the 65nm Cell B.E. that runs cooler and more efficiently (200 watts cut down to 135 Watts) Less than $0.09 USD KWH I recokon
Long story short, your paying more to fold on your PC. If you continued your regular routines, and folded on the PS3 instead your electricity bill would be cheaper.
That'd really sway me if that was the whole reason why I don't use the folding at home on my PS3.
My PC is already on, I leave it on to do an anti-virus and anti-spy/adware sweep while I'm sleeping. It's also on so that I can leave downloads up while I'm sleeping, or big file moves/conversions. Folding at home on my PC adds ZERO cost to my electric bill, because my computer is always on already, so it's just making use of any available processing power that I'm not using anyways and has no negative effects on me.
I do not leave my PS3 on, or anything else in my home theater turned on all the time; so leaving the PS3 on all the time would add in an extra cost that I don't feel like incurring. I'm also not sure that I have that chip set, so I'd be paying more per hour to leave it on.
I use my PC all the time, which is why I leave it on, so that I don't have to sit and wait 2 minutes for it to fully boot up. My PS3 sees only occasional use at the very best(and is usually on for maybe 20 minutes a week to check the PSN and get turned off in disappointment), so there's no reason for me to leave it on other than this program, and that's no where near a good enough reason.
Edit: if you have a PS3, don't use a PC that often, and you're really into folding @ home, then yeah, the PS3 is obviously the way to go. There absolutely is nothing wrong or immoral with not using the program though, for any reason. I don't feel like paying more for a program that makes more efficient use of a system's resources on PC(in terms of when/how it's running, not processing power) than on a system that I rarely use and don't have much value in. Regardless of my reasons, I don't use the program and there's nothing wrong with that choice.