It depends on where you are and who you're competing against. In smaller markets, or if you have unique skills, employers searching will be finding a small number of employees, and won't care about Stack Overflow participation. So for example if you know lisp and want a job in Austin, you're competing against exactly one person, who only has 424 reputation, so you're pretty likely to get a call anyway even without StackOverflow reputation.

If you are competing against more people, say, if you're looking for a Java job in the Bay Area, it may work against you, but I doubt if it will make the site useless. In my own recruiting using Stack Overflow careers I found plenty of good people who didn't even use Stack Overflow and I still contacted them because their CVs were great.