20$ a month? Absolutely that can be a lot of money if you have to make car insurance payments, house/rent payments, grocery bills, electricity/water bills, phone/internet bills, etc...
Not everyone has a lot of disposable income and even if they do have a bit of money they might not want to spend it on a forum they only see as an interesting hobby. Sure
you might want to spend money, but not everyone does.
Though anyone who says paid hosting is the only way to go would do well to remember there are legitimate reasons why people use free hosts. Whether it's they don't have the technical knowledge/background enough to go self-hosted. (Remember not everyone is good with code or servers or things like that. Some people can't touch code but can run a forum just fine if they can stay away from code. And maybe they don't have any interest to learn how to do that.)
Plus the money can be an issue especially if a site takes off and bandwidth usage jumps drastically. Suddenly having to go from 10 a month to 40 a month can make a pretty huge effect on someone with a limited budget. So not having to worry about that potential risk is a great relief.
Sometimes people don't view going self-hosted worth it if they're not even sure if their site is going to take off. Let alone if they're going to have any interest in that site 3 months later.
I would also point out that free hosts can be pretty reliable, more so than some of people's self-hosting experiences... How many of you have had sites killed because your host went belly up? Or they suffered tons of downtime and speed issues?
For free hosts though, I would recommend Jcink Forum Hosting over Zetaboards for
these reasons. I also recommend people to stay as far away from proboards as they can because proboards for an admin is extremely limited in features and they trap you with them by not letting you get your database legitimately.
I think there are benefits to both free hosting and self-hosting so it is up to the individual to weigh the pros and cons of both then make an educated decision.