Mark Suster (Build-Online/Koral/GRP) has a good post on why it's reasonable for founders to take some cash out in later-stage financing rounds. Mark has more experience of running heavily VC-backed companies than most people, in both Europe and the US, and gets both sides of this particular debate.
I totally agree with Mark and I think his 'strawman rules' for working out how much founders should be allowed to cash out and under what circumstances are very useful. The only comment I would question is whether Mark Zuckerberg deserves $20m, but then thankfully that dilemma doesn't come up too often.
Mark's key observation is that in considering partial liquidity for founders, both investor and founder need to take into account the main objective: to align incentives. This requires evaluating not only how much value the founder has created in the business already, but also his current financial position, cash taken out in the past, and the need for appropriate lock-in on the founders' remaining equity.
As the growth equity investing model develops in Europe, more founders will seek partial liquidity. Nearly every investment we make involves some measure of cashing out existing shareholders, so we work hard to get this balance right.