I agree that being open source is a plus for the user. As a TextMate user, I feel a bit (but only a bit, TM is still a nice piece of software that works well) cheated by the lack of development. If TextMate was open source, this wouldn't be a problem.
Of course, there's always the problem of reconciling money and open source...
It's a shameless plug, but I've been exploring possibilities of doing such reconciliation: http://open.hardcoded.net/about/
. It works rather well, and I'm sure it would work much better with a product that is targeted at developers (because most developers understand the problematic of intellectual property).
I haven't tried ST yet (but it does look like a great alternative to TM (Python!)), but I don't like the idea of investing money in a closed source application again. What if the developer (there's only one developer working on ST right?) dies? What if he makes so much money that he decides to drastically reduce his work hours? I suspect that the latter happened to TM's developer, and I don't like it much since there's nothing I can do about it.