There are an awful lot of threads on here where writers are asking for unpaid artists to collaborate on their comic with them. (I'm sure there are some artists looking for unpaid writers too - But the former is more noticeable). I don't mean for this thread to shame any artist who considers this and is into the idea, but to explain to the writers why this is an unpopular choice, and to new artists starting out why it's not the best option for them.
It's very similar to the "working for exposure" argument, that you can't use exposure/fun to pay your bills, and accepting one free job can very easily turn into accepting multiple.
But the most important thing is: If you're going to work for free, you might as well be creating your own content.
This concept is one of the best things you can do as an upcoming artist - if you're going to have an unpaid portfolio, make it stuff that's entirely yours! Flex your writing muscles! Tell your own stories you want to tell! It will be a much firmer foundation for your art career to stand on and help you develop what you're good at and what you enjoy as an artist.
If you're not sure whether you should be taking on free work for the experience or the exposure - You can expose yourself all the same on Tapas, and the drawings you'd be doing with an unpaying writer will give you the same kind of artistic experience as if you were to write your own - Yours might be better, even, because you don't have somebody else's expectations to abide by, and you have more room to experiment and improve.
I hope that's some food for thought for artists in that boat, and I hope that's a fair explanation to any unpaying writers who may not be getting replies on their threads and wondering what to do next - It's not that your work isn't good enough, or people don't want to work with you, it's just that free work isn't a great deal for any artist.
If you find a project that you love and want to contribute to and can afford not getting paid; go for it. If you have a close friend you want to collaborate with and you both just have to support yourselves; absolutely fair. If YOU WANT TO, for whatever reason, work for free; no one is stopping you. Power to you if you do enjoy getting involved with free projects and I wish everyone the best of luck!