ESE pays a paltry dividend(.8%) and is currently near the 52 week high. As one of the others pointed out, risk diversification is a big reason to not be in on the same stock as you draw employee compensation for. There's also the fact that as long as you're getting the maximum profit off of it from the employer(the 20% match), there's absolutely nothing wrong with locking in profits at whatever rate that is with the employer match and whatever their buying rules are. There's nothing wrong with locking in gains at rates that match Warren Buffetts.
For myself, I work at Intel(I'm a manufacturing technician), and so for me the choice is harder. With my dividend yield, the mastery of the executives at doing good things by shareholders, and the current price of the stock, I am holding my shares for the next year or two. After that however, I do plan on diversifying my risk outside of my company and selling most of my shares, but as I said, my situation and yours are different from fundamental stock aspects.