One of the biggest pains in the ass for me is when job sites make no distinction between recruiters and employers. A recruiter will list a generic job opening just to get a flood of resumes to store in their database for future use.
So I have the same concern about using careers.stackoverflow. Even if the fee is "hefty" for employers to register to see resumes, there are plenty of big recruiting companies who are more than capable of paying any fee that an employer is capable of paying.
Will careers.stackoverflow make any attempt to distinguish recruiters from employers? This would probably involve doing some background research on the company before setting them up on the system since recruiters can use a variety of techniques to create the appearance that they are a real employer. Fake job listings are the primary obfuscation method I have seen them use and it circumvents the primary method that is used on careers.so to keep recruiters out.
I would love to use the site but I absolutely refuse to allow the possibility of having to talk to these sleazeballs during my job search and will go to any lengths to avoid it - even if that means missing some job opportunities.