It really depends on your personal goals. If you want Jerusalem to continue to exist (eventually as an independent state), you'll just have to hold it until it's strong enough to fend for itself. If you don't really care what happens to Jerusalem, you might as well abandon it and let the Muslims/Byzantines conquer it. There's no gameplay reason to hold on to land that's causing you more annoyance than it gives benefits. Having 2 kingdom titles and more vassal Dukes gives you quite a lot of prestige, but that's about it. It's up to you if you think that's worth all the problems that weak, distant, and rebellious vassals/lands entail.