Welcome cosminus,

The starting stats of the generals (unit leaders) of your newly trained units are partly based on your faction leader's stats. If he has low command due to little participation in battle, so will the unit leaders of your newly trained units, if he has low influence, they will have low loyalty, with low acumen, they have low acumen, etc. The only exception to this rule are "hero" generals that will always appear with the same predetermined stats.

I have always found it is best to save the faction leader for only a few battles, but with the "heir and spare" in mind, use the heir to lead many battles. This doesn't mean you have to use the faction leader or a particular heir every battle but the faction heir can go out there and join the other generals on the front line. If his luck runs out then the "spare" is there in reserve.