I've done both paid (as a semi pro) and free (as an amateur) gigs, but something that always bothered me is the lack of respect you get when you're playing for free...
Let me tell you this anecdote:
We were asked to play for free (or close to that as we were hat paid) for an organic market during the hottest day of the year...
As I'm tea total, I struggled to get something to drink, first mistake from the organisers.
Second mistake was that all the market, including our PA, was electrically wired to the same plug running in two sections across a road!?!
We had a Bose L1 PA system that we bought brand new with our own money (it was the first ever sold in Belgium back then and it was a fame changer for us) and that we were using flawlessly since a few gigs.
Suddenly, after 2 to 3 hours of playing, the PA started to smoke and stopped working: in one word, it had burned.
The first reaction from the punters was: Why are not playing anymore?
The second reaction from the organisers themselves was: Do you have an insurance, because we have none?!?
So we packed everything back in the cars, after emptying the almost empty hat, while my fellow guitarist/singer kept saying: The duet is over, we can't afford to buy a new PA.
On the next day, which was a Monday, I called the store were we bought it explaining what happened and they called Bose directly who agreed to repair or replace it for free and even offered us to come directly at the importer place, which wasn't that far for us
We came there on the next day and the guy in charge confirmed that it would be repaired or replaced for free as it was under warranty at the end of the week, because we needed it every weekend!
On the Thursday afternoon I got a phone call saying that the PA was repaired and that we could come and take it back for free.
On the next day, Friday, we were there and the guy in charge of the repair section explained that all power amps of the L1 had simply fried due to a huge surge and that they had never seen such a thing happening in a normal use: So, I explained why it probably happened with the "main" lead coming from a house across the street with another lead that was connected to the first in the middle of the road where a car smashed it creating a huge short cut and power surge at the same time.
They laughed a bit and advised us to have our own heavy section lead extension and for our own usage only, which we did.
Since then, we are ALWAYS paid to play, because if the organiser makes profit, I can't see why we couldn't be paid: After all we are not jesters or minstrels living on leftovers.
Lesson learnt.