Grond will not be summoned anywhere in the map but near Gothmog's range.
No I understand what you mean, but if Gothmog is near the enemy fortress assaulting it, then Grond could be summoned right in front of the enemy's fortress.
I could say the same thing about Isengard's mine power which can be used in the middle of the camp and causes destruction. Remember that Gothmog has to be level 10 to use the ability.
To be fair, I think there can be a difference in power between an ultimate-level spell book power, and a hero's level 10 power (Sauron excepted). Also, the mine power is a one-use attack, which can be used to destroy a single, or a pair of buildings. It is extremely powerful, true, but Grond can move around and cause a great deal more damage. I suppose the question would be how long can Grond be summoned for? It would have to be summoned long enough to have some effect.