Jetsetter,
This was a bad thing during 80/90s.
It is the mightmare scenario if the enemy has your frontline combat aircraft that is in totally 'working order'.
A aircraft which 'you' still use.
Or do you think that the Soviets were happy about USAF flying MiG-15/17/21/23 or now MiG-29 in the Groom Lake?
I think that they were very mad.
(there has been also at least Su-22s, Mi-8/24s also.. AN-12s.. etc flown by USAF.. some of these were captured in covert Delta Force strikes in places
like Libya, from where the first 'USAF' Mi-24 came.. i belive that 3rd thieft attemp was succesfull.. but i dont know what went wrong with first
2..)