I wanted to contribute a little to the implementation of such units.
I agree that their link to the Tower of Orthanc justifies their position as defensive creep-like units. I was thinking that, since they wield spears, they could serve as an early-game counter to cavalry rushes (but if the Warg Sentries on defensive build plots are meant to have this advantage, this might be overkill). Still, if the ability to temporarily summon these units were linked to the Wizard Tower, their usefullness would be restricted to mid and late game. At this point in every game castles and camps will be more often under siege and I always felt that Isengard's bases were very empty, left without defences so that enemies can just run inside the camp (an aspect that can be prevented in Mordor's castles with the Gatekeeper expansion). The Warg Sentries will improve this situation with the next patch, but the Dunlending Guardians of Orthanc should prove more durable.
Then I realised that Saruman has this useless ability when in his upgraded Wizard Tower at level 3: Power of Speech is supposed to upgrade recruitment buildings but with the centralised system of upgrading since 4.0 this has no utility whatsoever. I would suggest to implement the ability to summon Guardains of Orthanc here, because they exactly emphasise the aspects of defending Orthanc, Saruman, and the entire base when under siege. The ability could be done as such that Saruman orders the Guardians to protect a structure inside the base from enemy attacks, which is more useful to the extremely defence-oriented Wizard Tower as a whole, and improves Isengard's abilities in times of siege. Of course this ability should be on par with Saruman's ability Power of Speech on ground, for example by summoning more Guardians, making them as strong as beroic units (or like Denethor's summons), making them uncontrollable but permanently guarding the structure till death, or increasing the number of summoned units (between 5 and 15).
In general, I am FOR the concept. It will be a great improvement to Isengard's defensive play.