Not only could Crusades and Jihads suck up troops (a very nice way of weakening your neighbour for an attack) but it could also at times suck up generals or even princes.
I was so unlucky to have my best prince taken from me by a Crusade.

Then I began wondering if he could in fact become king, for he retained his crown (meaning royalty for that faction). It would be nice to see my family on the throne of two factions.

Also Crusades could travel over other factions' ships, so you didn't need a line of your own ships, just a line of ships in general (as long they were catholic).