yeah, I've never seen a real study that studied if the power surge of a power on cycle does more damage over time than leaving it on... One thing about leaving it on that does shorten it's life is power surges from storms. I've lost more equipment to that than anything else. (Course that's hat surge protectors are supposed to help... maybe).
But in the end, especially these days, it all comes down to power use. Electricity = $$ and in some cases it's significant. If a machine NEEDS to be on 24/7 then it should be left on 24/7, if it doesn't then it shouldn't.
Not to mention tha anti-greenness of just plain pure power waste which we should all feel a bit bad about (Till fusion is all up and running I guess).
I've got 8 active machines in my house, if I left them all on 24/7 it'd break me
As it is there are only two, a WHS (server) of mine and a linux server of my wife's and I wish we could agree on getting hat down to one, but oh well. That alone probably costs us about $30 a month or more.