Final Fantasy XIII-2

[Neilikki]

As I have said, he's not just the only figure who's part of both teams

They've already said that after the development of FFXIII, a large number of people that worked on it, went to help with FF Versus XIII, and also that producing a sequel to FFXIII didn't require so much human resources as the previous title. Demanding that all people just stop doing whatever project they are doing and go help with Versus is just unrealistic.

And though you can call him as "just" a character designer for the XIII series, he tends to work at only one game at a time.

So, you think that after he submits his designs, he then just waits after the entire game is completed, 2-3 years later and he doesn't do anything in the meantime?
No, it's obviously not the case, even now, he works simultaneously on both FFVXIII and new Kingdom Hearts title (and probably the next title after that too) while also working as a designer for other Square Enix Projects.
If he only worked once every few years he would surely starve to death by now.

Versus XIII has garnered such huge hype since E3 2006 yet we find out that it's barely even touched because Square's been busy with their other (lesser?)

Except they said it's already in the final stage of development.
Square Enix isn't a small company, that can only work at one project at a time
Just because you don't see much promotional materials, doesn't mean the game is untouched.
And, in the first place, it's not like Versus is some Square's messiah. All of the Fabula Nova Crystalis games gained huge fans attention and were awaited and wanted, and calling them "lesser" titles is plainly wrong.

All these have led to fans get disappointed in Square for releasing an unwanted sequel when they could have been working on what the fans think is their next best project.

Calling it "unwanted" makes you look very biased, you know that, right? A lot of people waited and still wait to play it.
Just because Versus is highly anticipated title, means that every title from Square until it is released is unnecessary?
That would be true if only people that anxiously wait for only Versus could buy their games, but fortunately, they have audience beyond that.
Will Versus sell more copies than FFXIII-2? Probably.
Should they give up on those potential 2-3+ mln sales XIII-2 will eventually bring them worldwide? Why would they?
And I remind you again, that working on FFXIII-2 didn't hurt Versus, because refer to point one.

tl;dr Stop looking through Versus googles, it distorts the vision.