Do you think software products are more useful that don't present themselves as an "all in one" solution for something? For instance, did you consider, then decide against rolling Highrise into Basecamp?
You have to draw your boundary somewhere. But for Basecamp, our whole pitch is that it's an integrated solution with everything you need to communicate and coordinate within a small growing business.
Highrise is more geared towards sales, leads tracking, and such. Different domain.