It's certainly a difficult question. As to the specific instance(s) of halving/nullifying/absorbing Ultima Beam, it seems that only a link between an Elemental materia and any materia without a specific elemental property (for example, HP Plus and Transform would also be suitable) is actually necessary. I'd like to think of this, actually, as some sort of a "WEAPON element," but there is no obvious documentation refarding the materia set-up being effective against any other WEAPON-unique attacks (Emerald Shoot, Diamond Flash, etc.). I'll be sure to test this at some point, but until then I'll have to assume that the phenomenon is unique to the Ultima Beam attack.

In any case, I would certainly agree with the assessment of Ultima as an assault utilizing raw Spirit Energy. I think, however, that Ultima Beam is more descriptive of the WEAPON using it rather than a variant of the Ultima spell. Perhaps, though, the view of certain magicks being some sort of combination of elements applies most effectively in this situation. While the spell Ultima is simply a raw blast, Ultima Beam, used by a creation of the planet, may be more refined. Here, the characteristics of materia of no specific property can be applied in a focused attack. This would allow the attack, however, to be guarded in the same manner as elemental techniques.