I assume the decisions about where to use PA is made by the festival organisers, who arrange for it to be provided.

In a small venue like Carinas it is perhaps debateable whether PA is necessary. I have been to just to provide volume, but to provide balance between instruments and to allow vocals to be heard. When the place is full it can be difficult to achieve this without a PA, especially with several musicians. There again, having PA may encourage some people to chat, whereas in an unamplified concert they might be more attentive. It is a difficult balance, but on the whole I would say it is better to have PA than not, provided the overall volume is not too loud. It should be there for reinforcement and balance, not volume.

A sound check is absolutely essential. This is when mics are set up, technical glitches sorted out, and the sound balanced and EQd before the audience arrives. Equally important from the performers' point of view, it is a chance to get the foldback mix right so they can hear themselves. Sometimes this takes longer than planned, but its better for audience and musicians alike if problems can be sorted out before the performance starts, even if this means a delay. Sometimes things go wrong during the performance. Ideally the sound engineer shouldn't be too intrusive, but sometimes mics have to be repositioned or faulty gear replaced.

To complain about someone having to leave an event because they have a job to carry out elsewhere on behalf of the festival seems to me bizarre. The festival could not function without a large number of people - musicians, stewards, administrators and sound engineers, among others, who are willing to give up their time, and in doing so perhaps miss events they would like to attend. Some of them may be paid (probably not much), most do it for nothing more than a festival ticket, which they won't be able to take full advantage of because they will be working. If you spend most of your time in places like the Volly or in pub sessions perhaps that won't affect you, but the main festival couldn't function without them.

As for thanking the sound engineer - the first rule for musicians is "don't piss off the sound guy".

As someone who performed at this year's Sidmouth, I can say that the sound engineers worked extremely hard, in some cases covering several different venues between 9am and late at night. For my band anyway, our soundie did a great job - thanks Tom!