But then it's hard to set a definite timeframe. What you want is to incentivize sticking together as an effective team to reach a point of maturity, to maximize the chances of getting there and thus the options being worth anything. It's probably more about size and valuation. That's what was nice about effectively tying it to IPOs (when IPOs happened earlier). Going public was a decision that the board made when they company was "ready".
The engineer market in SF right now is very highly heated, and this kind of incentive makes an offer from pinterest far more appealing. That and people are starting to compare notes and make articles on medium and others really spelling it out is probably hurting recruiting and this is coming out as a response.