Leona who? I guess anything's better than the norm... I've never cared for the melodramatic "theme songs" they've been clumsily inserting into their games in the past few years. It's all trite, poppy Asian shit, gross!
There has only been a couple times where I enjoyed it (Kingdom Hearts) or didn't mind it (Final Fantasy VIII). It is a trend I wish developers would get out of. As for the song in Final Fantasy XIII, I listened to it on youtube I it feels like it will go into the tolerate category. But we'll have to see how well they place it in the game first.
I think the game theme song thing stems a lot from anime personally, where the creators select or have a theme composed and have either and opening and closing themr or both. If the single is to be marketed, it's marketed well.
So say a new single comes out, the single has the anime all over it, and the ads and anime splash on about the single. Unlike a lot of western TV, the opening/closing is often around a minute and a half and therefore is able to sell the single.
So with the advent of FFVIII's theme addition, it then became the norm for the series to hire someone to sing/write/etc a theme song for the title. With this sort of direction you could argue that in order to really sell the song to the US and Europe you really need to select a song that you can actually purchase over here. Even with the advent of iTunes and other online legal music purchases it really comes down to the fact that you're more likely to buy something that's actually designed for your market than something that was shoved in that may not make sense.
This is what I meant by is this a 'real localisation' - by choosing a song by an English singer Square Enix can pursue the idea of cross promotion in the West and fill more company money bags (for themselves and all others involved).
Is this at the expense of the original product (ie: the game)? Given what Matt said probably not for another people, "poppy Asian shit" doesn't help a lot of people unleash the theme for a given title in the form of music. Anime itself generally these days retains whatever themes were in the original, though some choose to create an entirely new one.
I guess the answer is simple, if people like Matt who don't like the "poppy Asian shit" find the Western equivalent is either the same or better, no one really loses out.