>Either you must get lucky to use it (random air pick) or you need to design a god around casting these spells (which, by the way, is not encouraged by the misleading (well: downright false) "flavor text", in which the summons are described as being "only" summonable by the CMs, which is not the case). That seems really very bizarre.
Actually it's more of a "imperative text" than a "flavor text". It does not fit to have other mages summon them. If you read the desctription you might get a feel for what is right and stop using other mages to summon the servants
>If it is purely a question of balance, why not make them available to celestial masters and more expensive? (*IF* it is a question of balance.) Or make them weaker. Otherwise, I can't see why the horse servants should require 2 air, nor why the gardeners should require a path that the national mages rarely get.
It is a special thing and a flavor thing. Gardeners and servants poke the earth in the heavenly gardens. Celestial Soldiers defend the heavenly abodes in the clouds.
The special thing is almost a balance thing but not entirely so. Celestial masters are rarely into earth (a flavor thing). We didn''t want to give them earth, but we wanted the servants to be earth based. The Soldiers are very powerful and should not be easier to summon. Thus the current solution. It also fits that you only summon one gardener and that you rarely build armies around them. Unfortunately (as I'm very fond of them) they become rare.
There should not be any problems with the soldiers as they cost more. When you get the air pick you might not be able to cast the spell anyway.