Now, the benefits are often illusory (see: pretty much anything Conde Nast has acquired). But sometimes it's actually true - particularly when there are economies of scale or actual (I hate this word) synergies.
This is pretty much the Google promise, isn't it? "Get bought by Google, get access to infrastructure you could never afford on your own!" This makes sense for some businesses, not so much for others (FWIW, 37s is probably in the latter category).
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?