Would never pay for such a thing myself, but I HAVE spent an inordinate amount of time on numerous occasions, standing in line behind couples and families, who have argued at length with the check-in people, in their quest to sit together. I have seen it take 20 minutes or more to check in people being awkward about this and they always seem to be the ones in front of me
I assume this is a charge paid in advance? If so, and it gets rid of this problem, I am all for it.
That said, I have on occasions, benefited from the odd seat upgrade, by being kind enough to move, so the aforementioned can hold hands. So, perhaps it is not such a good thing....?