Depends on the person and the company - I know DH's big boss is frantically trying to get the company heads to give him a promotion, or at least is saying that. He also knows if DH is going to go, he will (that's how he came to his current job, the old one wasn't keeping their promises).

However, if DH goes then they lose a big contract that DH is on. I have seen people who don't pull their weight threaten to go and DH's only words have been "at least I don't get their mistakes on my desk next week."

I have also used the threat of moving jobs before - gone out and found a job and moved to it because work was messing me around. However, I am now in a job where most contracts state you have to say if you're applying for other roles before you do so: personally I think it's only fair to let the company know they have to pull their socks up or look for a replacement soon, especially if you are a valued worker. (As I clearly wasn't for my earlier job!)