Who's the sucker now?
The biggest disadvantage is that big companies that are willing to buy aren't risk takers ... they'll either dilute your idea, or kill your project. Google is my kind of company, but they've killed at least a couple of startups they've bought already.
Of course, I wouldn't place 37signals in the same league. But he does have a point ... if your company can handle the growth without external resources, than why sell?