Yep its manouverable and goes mach 2+. With top electronics it's still a threat.
A threat, maybe, but so is an unsupervised 7-yo with a BB gun hanging out near a runway. Even with full upgrades I believe (I'm not very knowledgable in cold-war era soviet jets) it's listed as only being able to carry at most one bomb or DT, two AA missiles or DTs and a 30mm cannon. I doubt even with upgraded avionics it's much of a threat to our country these days, compared to our current stock of hardware and modern electronic warfare and jamming capabilities.
Why would North Korea or China stage it?
ack-ack
N. Korea definetley didn't stage it, heck they're probabley the sole reason why it crashed short of its destination in the first place, as long as N. Korea has had a history of officers defecting in aircraft they've had a history of never fueling up aircraft anymore with enough fuel to make it far beyond the border and the officers that fly them are aware of this. Similar to the policy N. Korea has about it's border crossing guards, they mostly watch and police each other rather than watch and police the border, they're always paired and always standing at attention and staring at each other, watching and waiting to stop their buddy if they decide to make a dash for the border.
What I think may of been staged though is that nobody survived the crash. Knowing China's relationship with N. Korea, if they admitted to having a defector then the N. Koreans would demand he be turned over before any useful information (if any) can be gathered from him, and then they'd likely kill their defector once reataining him. It may also be the best thing for his family back home if he has any, let them and his government think he's dead so there's less repricutions handed down to his imediate family. So, given all likely choices to a N. Korean MiG-21 pilot for defecting, I'd opt for the "fly as far into China as ya can, ditch/crash and then beg/offer everything and anything you could to the Chineese in exchange for a meal and claiming me dead" plan too.
China certainly doesn't want to give the impression that it's okay for North Koreans to come on over the border, whether it be a military officer trying to use a MiG to sneak over or civilians crossing looking for food and a better life.
ack-ack
Assuming the defector held some intelligence value to China or not, yours is another good and very valid reason for China to play the "defector? what defector?" card.