Since we can assume that there isn't any sort of limit on how many souls a Dragon or Dragonborn can hold at one time judging by the player-character and Miraak's endless absorption in Skyrim, does this pin those with Dragon souls as one of the most powerful soul capturing tools known to exist?

I mean, think about it. That's potentially hundreds of the souls of lesser Aedra that can be held all at once with no known consequence. If those souls could re-purposed one could generate tons of power. We don't exactly know if it actually is possible to use the soul for anything but learning Words of Power, but Miraak does use the souls of Dragons to entirely regenerate health and the Last Dragonborn can use the souls at will to learn whatever Word he/she chooses. This suggests that the souls could be used for even more, like powering extremely powerful Dwemer machines. I'd like your thoughts.

While the souls could be powerful, there is only one entity that can absorb or use them. So, they would have to strap the Dohvakin to a machine to get the power, and I don't see my Dohvakin putting up with that.

Beyond the Amulet of Kings, the only way to strip a dragon of its soul is to have another dragon present. Otherwise, it seems to stay with the corpse.

An alternate explanation:

Dragon souls are fragments of Akatosh's soul. Even a Dragonborn's. What you are seeing are two fragments recombining into a larger fragment.

What would a dragon do with any dragon souls it absorbs anyway? Only thing that comes to mind is a "Highlander"-scenario where every eventually, every single dragon soul in existence will have been absorbed by a single being (Dragonborn or Dovah), unless Akatosh feels the need to "create" more that is

There is no Highlander scenario. I won't dismiss outright, but the only thing even marginally resembling this theory is an excerpt from There Be Dragons, and we all (more or less) know how Hjalti/Wulfharth/Zurin really earned apotheosis.

That dragon becomes Akatosh by the "fragments of Akatosh" theory.

Mantella anyone ?

http://www.uesp.net/wiki/Lore:Mantella

I guess PlayerBorn should be thankful that they aren't the dead party in any Enantiomorphc Events, and that no one had another Mantella nearby.

I don't think Mantella would be able to absorb souls on its own... You know Tiber would fall as a first victim to it

Hence why I said "aren't the dead party" in any enantiomorphic event.

Soul trapping usually involves ruination of the physical vessel first. Even absorbing dragon souls requires that.