Yes, it can be effective while you tech up your other provinces. In my current campaign, I have retaken Milan, but I simply put the minimum in it (fort and tower) so as to reconnect Venice to my empire. Often when you retake these buffer zones, they'll often not be your soldier building provinces, just mere 'filler' provinces. I plan to do the same with Anjou and Aquitaine when I finally force the French out, though I wouldn't mind getting another ransom (if the French have any money left) !

I never look upon most provinces as necessary. You have to look at a long term strategy of rip and smash, and leave, and on to the next province. Of course, if you can grab a jewel, try to keep it intact if it fits in with your line of defense and your general strategy for expansion.

Of course, you may use crusades to achieve this end as well, I have yet to use a crusade though I will probably start soon to crusade so as to gain GA points. Crusades are another example of getting others to do your dirty work with your general in charge. I hope to soon crusade as I tech up and look for new rebel lands to seed !