I can't help but agree with that assessment. Paladins are just way too important, cost-effective, and useful for pretty much any party. Besides, if you team them up with other units properly, they will get you through any battle. For example, I love using Paladins + Knights. Paladins have Physical + Magic resistance while the Knights have Physical + Fire resistance. This way you get some nice flexibility for most of the battles. Enemies do physical or physical + magic? Use paladins + reserve ranged stack. Enemies do fire damage? Use Knights + Paladins.
The army that they don't fit in is the pure undead army, pure demon army, or an army of level 5 units. Paladins can only resurrect anything that is lvl 1-4, but that's barely a problem, since there are tons of great lvl 1-4 units
EDIT: Oh, of course. Plants and mech units too