Even when Cheechoo was scoring 54 goals, no one was saying he was one of the greatest players ever. He was a fantastic finisher, but he lacked the puck handling and skating to dominate a game. Marleau has dominated games throughout his career. Not consistently, but he can take over a game, and he's even got solid numbers in the playoffs. He scored two hat tricks in a series against Colorado, and that was before Thornton even came to SJ.
Now Marleau is a great player, but who knows what will happen in the next few years? If he plays the way he has this year and last, then he deserves to be considered one of the best in the league.
You can make the argument that pretty much every great player has had other great guys around them to elevate their play. Ovechkin has Semin and Green. Crosby has Malkin. Lemieux had Jagr. I don't think Marleau is there yet, again he was very inconsistent in his early career, but if he keeps up this type of play for a couple years, then Thornton-Marleau deserve their place.