[st.patrik]

Well early summons aren't necessarily going to be so incredibly hot, but summons in general are a potent (some say too potent) force, for several reasons:

• they don't cost upkeep (in most cases)
• many of them are more powerful than recruitable troops/commanders
• you can summon units in a castle under siege but not recruit units
• gem income is not affected by unrest or negative events (unlike gold)

there are probably more advantages, but that's what I can think of off the top of my head.

However summons, like all troops, need to be used thoughtfully in order to be effective. Many of them have niche uses, like the Hawks summoned by the Call of the Winds spell - very useful for attacking provinces you can't reach with your armies, to sow confusion in the enemy's heartland.