My own take on the whole, "Lightning," thing, is that its annoying, but doesn't ruin the game for me by any stretch of the imagination. I can handle allusions to other games, and I wouldn't have minded Lightning armor via some optional quest, but to transplant the entire character is a bit annoying. As a whole it shows a bit of disrespect for the game the developers are building.
To be fair though, they did do the same thing with Cloud, in Final Fantasy tactics. For some reason I actually enjoyed that, possibly because I actually like Cloud.
Simultaneously, I don't blame the developers themselves. The decision to throw in a Lightning cameo probably came down from on high, and the developers themselves would've had little choice in the matter. I'm not going to condemn the game, and all the good work the developers did, just because some exec in Square Enix's advertising division needs a swift kick in the hiney.