If an unknown company had launched this website I doubt it would have made anyone's radar, let alone have found a willing group of companies to pay $100/month. If it only works because 37 Signals made it and promoted it (as opposed to being an elegant solution to a problem) is it not a little exploitative?
Haystack's simplicity comes from a dearth of functionality and a small number of users -- neither of these are sustainable in the medium-term for the project to remain viable in its market (which, despite 37 Signal's claims, is pretty well serviced).
They risk damaging their brand by creating a feature-light product that doesn't exhibit any capability to grow beyond a small number of portfolios. They could well find an increasingly dissatisfied group of customers demanding features and results for their $100/month. It will be interesting to see how they keep these people happy while continuing to look after their other interests.