It's rare when you run into a situation where even the power amps and mixers need protection, but a small computer UPS is a godsend for arrangers, especially those that are prone to crashing on brownouts... If you load samples before the gig (as you are forced to because of glacial load times!), again, a small UPS just on the arranger (and any other low load stuff like effects, laptop, iPod or CD player etc.,) can save you from an embarrassing 'dead' few minutes.

An industry strength power conditioner is a bit of overkill for 99% of what you usually find, but if you play regularly in a place with appalling brownouts, it MIGHT be worth while. Otherwise, just get a little computer UPS (some are no bigger than a large power strip) and DO NOT plug your power amps or lights into it!