When ES announced that the next game was AOM, there was an outcry about mythology, etc.
I am paraphrasing here but: ES did state that the new game would appeal to a new audience without turning off the existing Age fans.
By limiting the GP to a single use, old Agers are not being subjected to a spell-a-thon, and hopefully new players will come to enjoy an RTS game. It also will require some thought before casting it.
Do I use it to counter another GP? Do I risk the Summon Animal early when I may not get as much food?
And in the end, AOM is still an RTS, and I personally prefer battles being played out with armies.
Having unlimited GPs would be the equivalent of TC wars in the prior Age games.
Who wants an RTS that is all about GPs and not battles? If that is your preference, I think your dollars would be spent better on an RPG or a game like Age of Wonders II.
Disclaimer:
All of the above is IMO.
"The devil and the priest can't exist if one goes away, its just like the battle of the sun and moon, night and day. The force of the devil is what we are told to fear. Watch out for religion when he gets too near."
"Living in a world of fantasy, I can hide behind what's real."
"Behind a smile, there's danger and a promise to be told, you'll never get old. Life's fantasy, to be locked away and still to think you're free! So live for today, tomorrow never comes. Die young."