Crusading still has a few advantages.

1. it gives your king influence. This has the minor benefit of improving your relations with other countries, and the major benefit of giving better stats to his progeny.

2. Crusading gives you free units that you otherwise couldn't get.

3. If planned properly, crusades can be marched through the territory of other Catholic factions, draining their troops along the way if they have high enough zeal (Precede your crusade with inquisitors to pump that zeal up)

4. I think I read that you don't have to pay upkeep on units that are in a "crusade stack" but I'm not sure about that one.