The Orthanc Guard is unfortunately competing with the Greyflood Wanderer for the title of 'Worst card in the Doomed deck archetype'. On the face of it, readying every time you raise your threat for Doomed sounds like a very handy ability to have - and I will note at this point that there is no limit on the ability, so if Doomed effects trigger multiple times in a round the Guard will ready every single time - but in practice it's only actually going to be useful if the extra actions you're gaining are worthwhile, which in general I'd say they aren't.
As a sidenote, one immediate issue is that if you are playing a Doomed deck then you may want to trigger a lot of your Doomed in the Planning phase, at which point the Guard probably isn't exhausted yet and thus his ability is wasted. Doomed from the encounter deck is certainly possible in the Quest phase but obviously you can't depend on it unless you've scried the encounter deck in advance. So if you want to be able to depend on readying your Orthanc Guard(s) you have to hold off on some Doomed cards (or Gríma's cost reduction) until later in the round specifically for that reason. Given the limited use of being able to ready him, is this really worth it?
So on to examining the use of those actions. The Orthanc Guard has 1 , which isn't exactly going to blow anyone away, and for combat he has 2 /2 , which will survive attacks from weak enemies or one attack from an average enemy barring shadow effects. And outside of truly universal effects which would work just as well on any other, more useful ally, those stats cannot be increased as there is no trait synergy for the Isengard trait (maybe we'll get some eventually, but I suspect if we do we'll also get some better allies for it than this guy). So maybe if you're playing a quest where you're going to get swarmed by 2 enemies then the ability to get extra defences from this guy in exchange for Doomed could be useful, but if the enemies are any stronger you'll be more at the mercy of shadow effects and you'll need to heal him between enemies. And/or you can get 1 extra on the quest in addition to that mediocre combat power. I'm not saying it's useless to have 1 extra and a chump-blocker in the same 2-cost ally, it's just that there are so many other allies you could include who would probably be more useful.
The one potential use for that readying would be if you have an effect which requires exhausting characters. A Very Good Tale is a decent one. Encounter card effects which exhaust characters can sometimes be devastating (when e.g. all your defenders/chumps get exhausted before combat) and this could let you get an action back. Otherwise these effects tend to require exhausting a specific character, or a character with a certain trait (Again, Isengard has no trait synergy), or a hero. Spare Hood and Cloak could be an odd one which might actually work out if you want to get into some shenanigans, but that's the only other option I can see. In general, while it seems like a decent ability, there are many better ones you could be using your deck slots for instead.