The UFC should be much more flexible in letting their fighters fight in other promotions. As long as they're not putting UFC belts on the line in these other promotions it really doesn't hurt the UFC.
To me, the best decision would be for Zuffa to require their belt holders to sign a contract for a title-defense in the UFC (with a concrete date for the fight)prior to allowing them to fight in outside organizations. That way, the UFC gets a guarantee that their belt holder will at least defend the title and not vacate the belt without a fight.
For example: [Note, I am just making up the dates and figures for sheer speculation] BJ wants to fight Sakuraba in Pride on July 4th. BJ signs a contract with the UFC to fight an opponent yet to be determined at UFC 50 for $50,000/$50,000. BJ bargains to have the right to refuse the first opponent but must agree upon the second opponent. BJ then signs to fight Saku for $150,000.00 in Pride and wins the fight. BJ is still under contract to fight on at UFC 50 on October 15th for $50,000/$50,000 against whoever the UFC puts in front of him.
If the management and the fighters can agree on a system similar to this, IMHO this will open the door for more highly regarded fighters making their way to the UFC. The fighters will have more options as to how they make their living and will be less inclined to flee the UFC permenantly for other promotions. At the same time remaining semi-loyal to the UFC and promoting it in other organizations for free.