Not saying this is always the case, but personally I always thought this was part of the reward given.
For example, it's not the same to be a developer that, even though might contribute a lot, didn't have to write the framework from scratch when there are so many unknowns, there a lot of tougher decisions, when things are not as clear and straight forward, responsibility is greater, etc.
Of course this depends a lot on the particular startup, but I've seen employees hired to do say, design, end up having to do sales as well, even if they don't enjoy it or they don't have the skill for it.
It must be pretty demotivating to spend your time at a small company busting ass to make the stock go from $20 to $21 while you look across the office at the early birds whose only current job description appears to be "sit around and cash out my truckload of $0.01 stocks at $20 every two weeks".
It probably would be; if an early employee really is contributing so little, they probably ought to be terminated (if they've got stock from early options, they can keep selling it off for as long as they want, whether or not they are hanging around the office demotivating other people.)