A pertinent question though:
What if there were others willing to sub for free if he would've requested a share of the pay?
The OP wants to get himself out there. He grabbed the opportunity, now they find out what he's made of and have him in their mind for future fill-ins.
If he does well and they know they can rely on him, they'll be willing to pay in the future. Or they should be at least, if they aren't idiots.
I'm inexperienced, but I probably would've done the same as the OP just getting into the local music scene.
I understand your logic,but in a great scale of things,this is the same concept of "internship" in another thread.
Getting musicians for free.
You are going to find hundred of people that want to "come out",and would do anything to get there.
But the fact you would play for free,doesn't imply people should abuse you.
You can see it the other way too : nobody else would play for free,this is why they asked the OP,knowing of his willingness.
Problem is that building this grounds,you won't have a future for your career,in the long run,with more and more people doing this,musicians will have no value.
An example : China restricted many years ago parents to have only one children,to control population.
Now they find out years later the consequence of this rule is that population is getting older and older,youngsters are scarce.
No work force.
What they do? : they propose parents to have more children.
Meaning the rule was counterproductive in the long run.
The moral of all this is : you keep playing for free,for exposure,you will build a name maybe,but won't have a chance to build a career,to make a living of your talent, because there won't be a market for it : always someone else will play for free.
This is NOT the market you want to be in,this will not buy your instruments,your strings,your amp,your car,your house,your children won't be going to college,and so on
But of course,people that has no purpose playing an instrument,that has no intention to live of their talent do not really care what the future holds.
I am totally against playing for free when everyone else is getting paid,as explained by the OP.