[Raynor]

I believe the subversion of the wills of all other metaphysical powers (those Valar weaker than Melkor—in other words, all of them) in the primeval Music and the subsequent corruption of all physical creation can only be corrected by a power untouched by this physical aspect, and ever unaltered by this metaphysical influence.

I think you touch a very important point here. I could only argue with the following. The physical presence of a 'mythological' power greatly increases its influence. We know for example that Sauron would have been greatly advantaged in a direct confrontation:

Originally Posted by Letter #246

In his actual presence none but very few of equal stature could have hoped to withhold it from him. Of 'mortals' no one, not even Aragorn. In the contest with the Palantir Aragorn was the rightful owner. Also the contest took place at a distance, and in a tale which allows the incarnation of great spirits in a physical and destructible form their power must be far greater when actually physically present.

We also know that the presence of the valar has a great effect on its vicinity - they effectively resist/removed Melkor's physical marring in Aman.

Originally Posted by Akallabeth, Silmarillion

For it is not the land of Manwe that makes its people deathless, but the Deathless that dwell therein have hallowed the land

Another important aspect to be considered is that actual association with great spirits elevates one immensely:

Originally Posted by Letter #131

A moral of the whole (after the primary symbolism of the Ring, as the will to mere power, seeking to make itself objective by physical force and mechanism, and so also inevitably by lies) is the obvious one that without the high and noble the simple and vulgar is utterly mean; and without the simple and ordinary the noble and heroic is meaningless.

Originally Posted by iii, Notes on motives in the Silmarillion, Myths Transformed, HoME X

In their association with the warring Eldar, Men were raised to their fullest achievable stature, and by the two marriages the transference to them, or infusion into Mankind, of the noblest Elf-strain was accomplished, in readiness for the still distant, but inevitably approaching, days when the Elves would 'fade'.

I would say that the two combined effects, over matter and over psyche, would be very good justifications. Plus, what a once in a lifetime opportunity .