I'm in a company with a similar system. I graduated from employee to "equal owner" - we've since had 3 more people do that - but also 3 owners (including the 2 original founders) have moved on.
We've made some mistakes along the way, and refined the system somewhat as well. What we realized (somewhat painfully) was that we wanted those working "in" the business to "own" the business. So while we have a vested system for earning shares, we also have a vested system for getting them back.
Once you leave, over a period of time, your shares revert back to the company. (we're a private company, not public, so the shares would only have any actual value in the event of a buyout, or dividend. The length of this vesting-out phase is proportional to the time spent in the company (with a cap). During the vesting-out phase dividends are paid out to "not present owners" in proportion to their share. To make sure this isn't completely manipulated we limit bonuses to the owners to the same % as what the staff get.
The idea is that while you're here, you're adding value. That value persists after you leave, but will becomes less important as time goes by.
The key thing - know how people get _out_ and agree on that before you start. Getting out is harder than getting in.If the rules are in place while everyone is still keen then they'll be fair. when it comes time for someone to move on, they already know the rules so there's no animosity on that front. By determining the rules _before_ you know which person will actually _use_ them you're likely to come to a very fair agreement. You know you could be on either side of the agreement later on. If you're negotiating this after one person has decided to use, then you've both staked out your camp and so both sides have very different goals - which leads to very difficult and painful arguments.
My partner once described business as a "marriage" and like a good marriage a pre-nup serves the interest of both parties.