The benefit of ancillary armor reps is that they give you a huge boost to armor health per cycle. You should be overheating the module to get the maximum benefit. The amount that you repair can also be influenced by:
ship hulls (ex. Myrmidons and their bonuses to local armor rep amount) or by
ship Rigs (ex, Aux Nano Pumps)
Most people who use them would argue that it’s very worth and almost a necessity. I would personally never fly an active armor ship without an ancillary. If you don’t think it’s worth it, you probably either don’t have the experience necessary to appreciate the additional bonus in armor rep, or don’t have the finances necessary to worry about it (and should just stick to T2 until you’re able to afford it).
Depending on the fit and the ship bonuses, it is possible that a ship has only 1000 armour hitpoints and your medium ancillary armour repairer repairs 600 of that. 60% in one cycle!
If that same ship happens to have 2000 armour hitpoints and your medium ancillary armour repairer cycles, it also repairs 600 of that. But now it’s just 30% in one cycle.
(Numbers may be inaccurate, no idea how much an ancillary armour repairer exactly repairs with nanite paste)
Ancillary armour repairers give better repairs in a short burst than a regular armour repairer, so if you want to survive the coming minute but don’t care much about what happens after that, and are willing to pay with extra nanite paste to do so, ancillary armour repairers are worth it.