Did someone mention a "why use this" question? My territory.
Caelum can move a support army quickly.
They can use mercs or indeps to form a hold-attack front line, wait for the enemy to charge it, then attack at the rear.
Caelum as an AI loves to pester everyone with random attacks of "Call of the Wind". With alittle more intelligent targeting and followup that can be one effective tool.
Caelum can play checkers with the map. Use your scouts or dominion to find weak provinces. Jump OVER rougher indeps like knights and longbow. Caelum has the ability to bring up both mages and priests to build all structures. Leapfrogging that way you can move farther and faster than anyone. And attacking you with an army such as Ulm would be doubly hard because they would have to fight you for a few, then indeps while you recover, to reach some more of you to fight.
Caelums ability to leap over provinces also makes it a good ally. One of the common complaints about "lack of diplomacy" is that you cant move armies across an allies area. But there are nations (Caelum is one) who dont really have that problem. One small stepping stone province will let you move across alot of allied territory and QUICKLY come to the aid of a friend.
You have fast access to one of my favorite spells Wind Ride to snatch enemy commenders just before a battle. For nations who tend to assign alot of army to a few commanders this can cause much of his army to rout if even a scout attacks them.