That's exactly my point, which is why it's dangerous. If the exploits were made public as soon as possible (full disclosure) then MS will have incentive to release bugfixes as soon as possible to the general public.
By taking the cash, I am sure you are bound by secrecy enforced by jail time. I see these increased payments as hush money to keep researchers quiet while they feed them to the NSA for zero day exploits.
Not to mention, the outsider is a complete unknown. Are they an upstanding white hat? Or are they the darkest of greys? You have no idea.
At least an internal employee is on your payroll, and has been screened with a background check. You don't get to screen which people get to discover vulnerabilities.
While that might decrease the total number of days in a year when there are unpatched exploits that MS knows about, it would increase the total number of days in a year when there are exploits known to hackers. I don't think that would make us safer.