On one hand, it strikes me as incredibly beneficial for the founders if the company is a runaway success.
On the other hand, the VCs are essentially throwing away their unlimited upside potential, and in doing so signalling they don't particularly believe in the company.
There's also the fact that the terms are very unfavorable for founders if they exit at anything below a stellar valuation, though one could argue the entire point of the game is to achieve an incredible exit.
Still, when incentives aren't aligned it's disconcerting.