Having listened to nearly all of Spolsky's SO/SE podcasts, I think the other factor is that Spolsky's strategy has been to control critical dependencies by keeping them in house. StackExchange runs on its own servers rather than in the cloud so that problems fall into StackExchange's queue not in with other people's at a cloud computing provider. If FogCreek had made Fogbugz depend on ChilliSoft, then infrastructure problems with Unix integration would have been fixed on ChilliSoft's timeline if they were fixed at all. Writing Wasabi meant that Fogbugz had a more predictable development schedule controlled and instrumented by FogCreek.