Earlier quoted context omitted.
17K for 7% isn't as raw a deal as you think when you realize that at an "acceptance" into YC essentially doubles your valuation to most seed investors.
Absolutely. Being vetted by YC certainly won't hurt your company, but it's still tough though when you've dumped $100k-$200k of your own money plus your time and energy (which is not insignificant when you're an engineer in the Silicon Valley), and you're giving up that amount of equity.
If you have no product and no track record, it's definitely worth it.
If you have something working, but growth isn't amazing, it's a risk because there's a chance you don't get the absolute most out of demo day due to timing -- so one might feel what you describe. That said if you blow up later, I'm sure they will do a fantastic job of getting you to the people you'd want to meet with - eg. Homejoy.
But if you come into YC with good growth (like doubling in size every month) it is without a doubt one of the smartest things you can do. You will probably never have as much leverage as you get while going through YC.