I like the fundamental idea. However:
1) This risks pricing the common, which is bad. It certainly isn't the same as preferred, due to superior rights (liquidation preference most seriously)
2) In general, you should discount equity vs. cash because equity, even granted today, is illiquid. Even if you claim you're willing to buy it back at any given time for the then-current market price, you're better off motivating employees by giving them equity than cash, all things being equal -- the employee can improve the value of the equity through work, but can only in the most absurdly indirect way increase the value of cash itself (by tearing up other cash...)
In practice, I love what Palantir does (or did; I know about this from Ari Geller on Quora, not directly). You basically get 3 discrete offers e.g. -- $130k/500 shares, $100k/1k shares, $85k/3k shares. It's essentially a way to reveal your preference and beliefs about the company (although, Palantir is now so big that an individual's contributions don't materially move the price.) (I'd personally be WTF at someone who picked the $100k/1k; either extreme would be defensible, but I'd generally prefer to hire the 85k/3k person, unless I knew he had high cash expenses like kids in school).