There may be cultural differences between Europe and the USA on this, but here's my perspective on this...

When you are applying for a job, as a candidate you are wanting to present yourself, your experience and your ability to do the job in question. You are actively shaping how you are presented to make it a simple choice for the employer to say yes, this is our new employee.

Employers are trying to identify who out of their candidates are capable of doing the work, have the ability to settle into the companies' culture and to hopefully avoid recruiting an employee that's causes more problems than they solve.

So, when I'm recruiting, I do not, and will not ask a candidate for their Stack Exchange identity, or their Facebook username, Twitter account or Google ID. I would consider all these as private personal activities, and would respect the candidates' reasonable expectation that these were not work-related issues, unless by their conduct they made them so.

If on an application, a CV mentioned their Stack Exchange identity, I would ignore it, other than noting that they use Stack Exchange, mildly positive for a graduate, kind of expected for anyone with commercial experience.

My interview process is about giving the opportunity for a candidate to demonstrate that they can do the work we are recruiting for. If they can demonstrate that, and look like a reasonable social mix then they will probably be offered the job.

I could see the Stack Exchange account being used like references, in that job is offered, subject to satisfactory references, but I'm still far from convinced that this is fair, and not undue intrusion into their non-work life.

If as part of an interview process I was asked if I use and contribute on Stack Exchange, the answer would be yes, but if asked for my user name, I would say 'I'll have to get back to you on that'. The reason why is this: I have never been employed to contribute to Stack Exchange, and until that changes, it's completely part of my private, personal life.

Now, consider what the effect would be on Stack Exchange if your profile became part of the interview process.

People would soon learn that you would have to have a pretty exceptional profile for it to be a significant factor in you being invited for interview, and that it's never going to make up for a bad interview. In short, its only effect will be to prevent you from getting a job.

So, just like you take great care about what goes in your CV, you would do the same on Stack Exchange. No comments, only very carefully thought-out answers, and if you were not 100% sure, you wouldn't post. Would you leave up a down voted answer? Or badly received questions? Of course not.

Stack Exchange will be the worse for it.