This is really refreshing (in terms of new troll models
) idea.
I am more for integration of this summon as power on lower level of palantir (something like vannila worm power). I really don't find this worthy as final palantir power, in other words, good enough to replace "Dragon Strike". As summon power this suggestion shouldn't be overpowered, is should be some summon of "medium strength", fitting for example 3th level of palantir powers (Something like eagles power), or maybe something of 2th level similar to "Mirkwood summon power" in Lorthlorien faction.
P.S. After all, in my opinion "Dragon Strike" as final power is still unique, no matter Goblin faction will have dragons or dragon hero as permanent units. It is maybe too much dragons in current ideas, but I don't find "Dragon Strike" power as redundant because of that situation. In my opinion it is more justified because of that.
I presume it is matter of personal preferences concerning dragons.