If it works for you, and you are comfortable with the risk, and it's in the bounds of your Investment Policy Statement, then go for it.
My caution is, if I'm understanding, it's still not risk-free nor along the lines of free money. Some employee stock purchase plans are essentially free money (buy at xx% discount, you can sell immediately), other then thane fluctuations between when you are granted, and when you can sell, that is almost risk free, and free money. The way I understand your plan, you get a 20% discount, but you are taking on five years of risk in holding a single company (either that it could decline, or generally under perform the broader market).
I wouldn't do it, because my investment policy doesn't include individual stocks, and the golden handcuff risk is too great (also, I've never managed to hold a job more than two years). But if you understand the risks, and it works for you, then go for it.