I don’t really like the term “power conditioner”. It could mean anything. For users like us, there are a variety of things that can be stuck in a power socket, and the usefulness varies (IMHO).
TSJ (can’t believe I called you ‘TJS’ 😳 apols) has a good surge protector. These filter out the spikes in an unreliable AC supply. Like Lee says, a good in-between for your prized head and Farmer Jack’s dodgy petrol generator if you’re playing “Cowpat II” festivals. I have to say that UK mains supply is pretty smooth and I’ve never had that problem. Others I know of (better players, tour more) in the US have had problems in some cases. Any experiences out there?
The RCCB (residual current circuit breaker) is even more useful at “Cowpat II”, especially if you insist on nasty Bulgin leads or other ‘original’ features that perm your moustache when you approach the mic. Personally, I rate myself as more valuable than the amp, and I chew as a player.
The other ‘power conditioner’ I can think of is a UPS, or uninterruptable power supply. When the mains goes AWOL, the UPS kicks in and lasts long enough for even a soundman to work out what went wrong.
Proper electric-heads please correct me, but I understand it that an RCCB protects you, a surge-protector can protect your amp’s transformer etc, and a UPS can keep you playing, given enough juice.
TSJ, to answer your Q, there may be pros who use UPS because big stadium gigs and $100 tickets mean a lot of refunds if the sound goes phut. It can get expensive, and not worthwhile if you can blame Farmer Jack for an outdoor summer special. A surge protector (yours may well have an RCCB built in, have a check) is as much as you need.
I think the surge-protectors and UPS were really aimed at the computer-user initially. I have seen some opinions that they (or a $500 mains cable) make somebody’s hifi amp “definitely” sound better. Large pinch of salt required when reading those ops.