Modric has spent large periods of his career on the left, as did Henry. Doesn't change the fact ones a central midfielder, ones a forward and niether are left wingers. So I'm not getting your point? Or are you just being pedantic?
On the off chance you actually believe he'd be as good a left winger, I doubt he'd be anywhere near as effective, seeing as his best football arguably comes when he plays with his back to goal and recieves the ball into his feet. He's quite clearly most effective in and around the box where his first touch and finishing are best deployed, resulting in him scoring nearly a goal a game this season. His best attributes aren't as effective out wide, and in addition other elements of his game might be lacking in that position, he's not slow, but he's not rapid either which would limit his effectiveness on the wing.
Pedantic for pointing out that a player has spent a large amount of his career playing, and playing well, in a position you claim he wouldn't be effective in? Have a word with yourself mate.
It was only 18 months ago that a large amount of people were questioning if he was up to playing as a number 9, and although he's since proven he's more than up to the task it's not the only position that he'd excell in.
Berkamp said just a few weeks ago:
“To be honest, I would still prefer to see Robin play a bit deeper behind the main striker so that he could pick his moments the way I used to.
"I think he could be even better in that role, but the way he’s playing now you can hardly blame the coach for putting him there.”
Klopp described his effectiveness:
“But Robin van Persie, wow, what a performance, what a player. He’s certainly one of the best in Europe”.
I’ve hardly ever seen a player who plays so deep in midfield and then is such a danger in the box.”
The point being that RVP is not just an outstanding centre forward, he's an outstanding footballer and is more that capable of excelling for Barca on the left if only they had the cash to buy him.