I don't think the problem here is with Google Ventures. The problem is that founders don't understand convertible notes with caps. More by Mark Suster here: http://www.bothsidesofthetable.com/2012/09/05/the-truth-abou...
If everyone is investing at $12mm, and an investor says "fine, but I'm special, give me $6mm or I won't invest", there's no difference if it's capped convertible note or equity (except that it's technically more difficult to do the investment as equity with different valuations; you can hack it by giving warrants or other benefits I think.)