Let's put it in these terms: Huorns are not as dynamic, if we may say so, as the Watcher, but they're not lone towers either, given their impact on masses of troops. It's like an in-between situation, and, believe me, the spell used to be much more powerful than Eagles or whatever akin spell
The topic does belong to this board for sure, being it a map-related suggestion. If you wish to debate Huorns further, you may head to the Lothlórien board, although I don't know how much I can be of help for you, because Lórien's spellbook is still being worked on by the Edain Team at the moment.