But those would be very difficult to summon. It would take a lot if magical talent and probably multiple summoners. Then you have to control that creature, which might not be the case. Guardians were a way for the Snow Elves to sort of mass produce a fully obedient army. The huge advantage is they they are just a suit of armor and won't tire and become difficult to beat. That's why I stake they are on par with greater summons.