Benefits are a "take each on their own merit" deal for me.
In reading the OPs words its a benefit with many bands and you get maybe 40 minutes. Not a lot of time to "build your fan base and popularity" Doing bennie shows gives you some of this but not a lot.
What bothers me most about bennie shows, and I've been part in way too many lately due to many old local musicians getting sick, is that the bands ARE the draw for those shows. And all you may get is free food and drink. That donation at the door only gets collected if the people come, and most of them are only coming because of the entertainment.
A local musician who recently passed here was in the hospital not long ago. They held a benefit to help pay his expenses. Fortunately I knew him enough to be able to help his family directly. I went to buy a ticket and the show was sold out in about half a day. So I went to the venue hoping that there might be some door tickets. None.
Long story short, someone had two extra tickets, and as I learned about it I was going to that person when a guy ran, cut in front of me, and threw his money at them for the tix. After that, he ran with his cell phone and started calling his friends.
He was not there because he wanted to support my friend. He was there because there was a great possibility that Bruce Springsteen would show up. He didn't care a lick for the cause.
Now, for things like cancer there are people who will support a benefit show to support the cause and have a good time. However, in most cases, as in my story above, when a group organizes a benefit with that many bands and its all music, they are looking to cash in on the band's following to raise the money.
In my recent experience all the bands got help with loading in and out, a backline of some sort was provided and the bands got drinks and food. Many of them just show up, do their thing and get out.
Yes, the bands are usually the ones who are asked to "donate" their service. The last time I was asked to participate for free I asked them who else was donating all their services for free. Of course they said the volunteers in the organization. I asked about the bar, and they of course said they were donating 20% of the PROFITS, meaning the bar was still making profit and paying the bartenders. Not working for free. All the vendors were getting paid. Some accepted a discounted rate, but they got paid. I asked them why that was. Blank stare. I asked them why should I give you not only my performance but access to my fan base for the day for free? Blank stare.
I only do benefits for people I truly care about or that I have known for a long time and are good to me. I only donate my time if they agree to have me prominently placed on the bill and program and I am mentioned in the radio advertising. Just because its a benefit doesn't mean that all the business goes out of it.
When I get asked TO DO a benefit that just makes no sense but I support the cause, I just go and buy my $10 ticket and attend. Much cheaper then donating my time. And, in most cases, I'll wind up playing with a band a couple of songs anyway.
Blue, in your case I wouldn't worry about this one. Its some weird bennie, that is for sure, and, unless its a cause that will bring about world peace, I can't see it doing a lot for any band. After all, as one poster said, its about the cause. Most people will not care who the bands are if its a good and big cause. If its not a great cause, most of the people who show will be your and the other bands fans and you won't really gain much if any additional following.
Hope that enlightens you.