This article should probably be retitled 'Working for half your market salary? You are probably getting screwed' - but then the whole thing sounds a little obvious, no?
If the company is strong, paying you appropriately won't make or break it. If the company really can't afford to pay you appropriately, you should be a cofounder.
Agree. You're only getting screwed because you're willing to get screwed.
You should always make sure you're getting what you're worth when entering any business relationship, and make sure that any gravy (stock options, bonuses, etc.) is exactly that. Gravy.
If the company is strong, paying you appropriately won't make or break it. If the company really can't afford to pay you appropriately, you should be a cofounder.