Ah, I see what you're saying. Landing investors can be strong social proof.
I suppose it depends on what markets you're targeting. 37Signals tends to aim at small businesses that would be more willing to embrace SaaS solutions. In Rework they admit they chase the common case because of the money involved. But targeting larger customers in more entrenched industries (such as health) is not as simple as making an attractive landing page + an edgy slogan.
