Owen Thomas at Business 2.0 has an amusing riff describing how VCs are like Mafia families. The analogy is a little strained, since traditionally, you paid the mob for protection, rather than having them "invest" in you. (Mobsters notoriously prefer shorting securities, property, and limbs.) Still, it's a nice way to understand why companies take VC money when they don't necessarily need the cash itself — what they need is the relationship and "protection" from the VC investing in a competitor.