The WC argument isn't nonsense at all. And it's not something which only goes against Messi, it goes against the likes of Ronaldo or Rooney too (and especially as it goes against Rooney, it's no surprise at all that a certain poster insinsts on calling it a nonsense argument, indeed it's very predictable).
There have been players who have performed well in tournaments' big games and at club level (notably R9, Rivaldo, Ronaldinho, Zidane, Xavi, Iniesta, Pirlo and some others). That doesn't mean that they are better players than Messi or Ronaldo, but you can't just ignore their performances in the biggest competitions in the world. A competition with immense pressure and attention, with the best players in it, an event which the whole world looks forward to every 2-4 years.
Then you also have a player like Iniesta who has been unfit and out of form before WC 2010 and Euro 2012 (in fact in 2010 he was out injured for more than a month so couldn't take part in Barca's end of season games, plus was kind of depressed because of his mate's death - the Espanyol player), but still gave a masterclass in those tournaments, especially in the later stages of the tournaments when it all gets very tough.
Now that doesn't mean Iniesta is the best player ever, but it would be extremely stupid to ignore that some players like Iniesta have managed to shine in tournaments despite not being fit.