I'm sure that sometimes it's the rush and sometimes it's the ego, but I find a huge problem with off-the-shelf code is that it rarely fits into what you've already written. It can be something as simple as having different naming conventions, to the almost-impossible-to-deal with problem of the third party code having a whole different idea of the place it should fit into the overall program's structure. That stuff is a tough problem, I think, and one there isn't a good/known solution for.