(Sorry for singling this post out; I just had to find some random quote from the thread so I could give my slightly different opinion lol)
I actually don't agree with the first part of this, which is that I think good artists should consider working with writers a lot more on their projects. There are so many great artists who do webcomics, but either they can't handle the double duty workload and the comic fizzles after 2 or 3 months, or despite the great designs and art direction, the story and dialogue just doesn't really cut it; if they had a writer they were working with (NOT for), it would have been better for them. I think writing is pretty undervalued in a lot of places and the time/effort is usually underestimated when people want to start a comic. Two people working on a passion project together can result in so much more if both people are equally dedicated.
However, everything else is spot-on, just like everyone else is saying!
The keyword, which is the big issue here, is the somebody else's story part of things. Which is the vast majority of writers seeking artists (and, honestly, artists seeking writers, as rare as that is). "Paying in exposure" or "Paying in profits" is always pretty awful to me, especially when webcomics almost never make any money at all. There's a naive assumption from the part of a lot of people that if they just find some random person, that person will be perfectly willing to work, for free, on some project that they have no personal investment in.
The only way working for free should ever happen (in art OR writing) for anything other than short projects is if all parties involved are actually passionate and interested about what they're doing, even with the expectation that neither of them will ever see a single cent out of it. Nobody should ever work FOR someone for free, but working WITH someone for free, if you find the writer partner or partners, is possible (to me).
So, everyone in this thread is saying exactly how I feel, except that I think there is a narrow, specific exception that shouldn't be overlooked.