There are arguments for both sides of this debate. Reasons for studios leaving gear on are
1) Powering down gear can mean losing the state of some settings unless it has all been saved to a preset. 20 years ago many devices didn't even have preset memories so powering down mean't it may be impossible to return to the same settings. This could be a big problem if you were in the middle of working on a track.
2) Powering up does stress electronic chips and powering up daily is probably worse for gear than leaving it on. For non-studios leaving it on is probably more wearing than powering up once or so each week. If you are having to turn something on and off a number of times each day then consider just leaving it on while in use.
3) The power bill for leaving these devices on is not huge so studios can justify any increase in their budget.
4) Old analog gear does become more stable after warming up for 30 mins or more so it may be better to leave this gear warmed up. This is not an issue with modern digital equipment.
5) If something does become too hot then it makes sense to power down when not in use.
[This message has been edited by Nigel (edited 01-25-2003).]