There are lots of temptations for sound technicians. If you use a small PA in a biggish venue, you have to drive it quite hard to make it heard at the back. A hard driven small PA tends to distort the 'natural' sound of the performer. As a result, the trend seems to be for more and more powerful PA systems. (Our band used to use a 250 watt Peavey rig; we now use 1500 watts of Bose L1 rigs) The problem here is that power tends to corrupt, as it were; there is a huge temptation to 'crank this baby up and see what it can REALLY do'. Big PAs are also able to accentuate the bass registers better than small ones, and this can muffle the sound or lead to that 'thumping bass' heard in Vauxhall Corsas everywhere.
Another temptation is to 'make the performer sound better'. This is really the performer's job, and practice is the only way, but sound techs often add lots of processing to the basic sound. Compressors, reverb units, vocal enhancers and the like have their place, but they don't reproduce the sound of a performer singing or playing to you unamplified from a few feet away in a quiet room. The result is often an artificial sounding performance, with a lot of the natural dynamics 'tuned out' of the mix.
In answer to Guest above, no, you don't need a microphone if you are playing to a quiet, attentive audience in a smallish room, and I would always prefer not to use PA unless it is necessary. Trouble is, as soon as a large number of people want to hear you, or you play in raucous venues, PA becomes a necessary evil if you want your music to be heard at all. Interestingly, the quieter you play, the more people listen as a rule. (This wasn't so last Friday; we played bluegrass to a corporate shindig in a huge marquee, and it wouldn't have mattered how big the PA was; the audience were only interested in getting legless and shrieking corporate in-jokes at each other. Paid well though). Also; it's not just performers who spend long hours preparing. Performers tend to do their set and go. The sound techs are usually there for long hours doing setup, sound checks and teardowns, they have to test and maintain the gear and carry heavy equipment to and from the venue.
The final temptation for both sound techs and performers is for each to blame the other for poor sound, when usually the cock-up has been a collaborative effort.
As I said, I'm on both sides of this argument as both a performer and sound tech. If you want to hear what my sound is like, come and see us at the Northwich Beer Festival in Cheshire this Friday!
Tim