Lets say Pakistan had no nukes and the US "engaged" with Pakistan for all the reasons you give. And then, in 2003 or thereabout, finds out that its not exactly cooperating wholeheartedly. My thought is that the nukes take some options off the table..e.g. the option cutting off aid and applying direct pressure...maybe nobody would have done that anyway, but even if someone thinks of doing it, don't the nukes inhibit that though immediately?..not because Pakistan could use one, but because the sanctions and pressures might work too well, leading to either collapse or open Jihadist takeover. To avoid either contingency 98.7% of analysts would opt for "more engagement" and while GHQ may not know many things, they do know this fact and use it.
Again, I could say (like Robert sahib) "good for them. they should protect their interests", except that i dont think that the interests defined in the Pakistani military-inspired national narrative are really in the interest of most Pakistanis.
And, as a US citizen and taxpayer, I do feel we shouldnt be paying for such shenanigans. I understand wasting trillions is our thing and Ron Paul is not going to win the election, but the thought still pinches...