1. Alleviating human suffering.
2. Scientific exploration and discovery.
3. Building tools or technology that enhances others' abilities to do 1. or 2.
Notice that "bug tracking system", "personal finance management", and "company collaboration software" are not on the list. (And no, I don't want to hear about a stretched analogy about how they actually are alleviating human suffering. Please.) Also notice "social networking" is mysteriously absent as well. (Edit: To be fair, Twitter has had a few days where I really think it is making a positive difference in the world. Crapplications on Facebook, not so much.)
The only excuse to not be working on one of these things is if you are working towards building some capital in order to do so. Flipping a startup is a great way to do this.
At the end of the day, all these neat software tools people are building are not all that important in the grand scheme of things. What they are good for though, is enabling visionaries to start to address real problems by removing financial constraints. I am hoping that Aaron will now turn into another Elon Musk now that he has got a big chunk of $140M to work with. While he is solving real problems 37Signals and everyone else who has a lower bar for what it means to be a real contributor to society will be running their "Italian restaurants" for the next 20 years.