The decision came down to an integrated suite of components more than anything (IP.Content for articles, the forums were quite good, blogs, etc). It provided a lot of flexibility and was more up-to-date than our old ASP technology. Migrating our custom database to this software took a ton of work figuring out what table was responsible for what.
At some point Invisionpower switched over to use CKEditor, an example of an open source editor coming to the rescue in lieu of their own in-house editor. What a total nightmare that has been and CKEditor still continues to not behave as well with IPB as we'd like. They tossed it for the newest version of their software.
To be honest, we looked for a total package rather than parts because cobbling together a system with a lot of parts that weren't originally intended to work together can get messy very very quickly. The editor is such a small example but the more parts you have that come from different vendors, the greater the potential problems that can creep up. There are certainly downsides to a single vendor (like when they completely rewrite the software from the ground up) however.. but the nice thing is that someone else goes through the tough job of making all the "stuff" work together.