(or just read the quora question on it http://www.quora.com/Venture-Capital-Firms/Have-any-top-tier... )
VCs get pitched all the time they can't afford to give unsolicited pitches more than a tiny fraction of their time for evaluation as 90% of them will be terrible. But if a pitch comes with a recommendation from someone they trust, it's much less likely to be a bad pitch so they can spend a bit more time looking at it.
Getting an introduction to a VC isn't hard, it's trivial to meet startup founders at events, etc. and most would be willing to introduce you to their VC if they thought you had a good idea.
If you're not capable of getting a warm introduction fairly easily, then that's a huge negative signal for the VC about your capabilities.