I've thought about doing that very thing - here's my problem with it though (and this 100% on me for being the way I am)... If I get into a situationship with another writer where they seem to have shiny new toy syndrome, I lose interest in writing with them very quickly. I don't necessarily lose interest in the line, but I lose my ability to trust that I will get enjoyment out of the writing with them. As soon as I get into something, they may want to move on to something else and I might lose a line that I feel invested in - if that makes sense. The idea of constantly having to switch lines with a person to feed their interest is not something that I am interested in. And I think that's why I find myself less inclined to reply to the scenes we already have going, when it appears they would like to put their time toward something else vs. what we already have going.
What they do with their time is theirs to choose. I don't hate on them for it. But if I am going to continue to knowingly write with the same writer, I'd rather be able to build something that is long lasting vs. changing every month and feeling like I can never be invested in anything we do.