I have to admire the way 37signals has grown over the last few years. Sure, they clearly don't integrate every feature. The user interface certainly works but has no iGloss about it at all. Pricing is steep and they hide the lower-priced plans. But it works: people still use the service.
If you're a coffee shop you concentrate on your coffee. If you're an electrician, you concentrate on the quality of your work. Adding extras like "nice cable ties" are irrelevant. 37signals are concentrating on their core functionality. When the day comes that the majority of their users require X feature and that feature becomes a norm in Project Management, Contact Management, Collaboration, etc then I'm almost sure they will react: why wouldn't they?