Preset classes are the only way to guarantee that everyone is offered the exact same opportunity than other players. I personally think that it's bullshit having a level 55 running around with higher everything on the map than level 12's, it just makes the game annoying and not fun. Now, if they want customization, go for it, customize the rounds in your guns, customize the color of your weapons down to the every last detail, customize the scopes/ironsights/triggers/grips/etc., customize away, young grasshopper! But, I still think that preset classes are fair for everyone and still offer a great deal of customization opportunities. Besides, it's already been stated that the new COD6 is not going to be a Call of Duty game, it's just going to be called Modern Warfare 2 (or who knows, maybe the teaser trailer for this game was wrong) but I would personally love to see this game not be a call of duty sandbox shooter #6. Another thing about the class system is that it makes every class unique, you have to have every class in order to achieve certain goals. Want to kill a tank? Bring out your engineer class or sniper class or specialist class (note that I'm using the class system from Battlefield 2: Modern Combat as my examples) Ground troops taking cover on a roof? Bring out your support class and call a mortar strike. Snipers giving you trouble? Why not bring yours out as well?! Need to assault that flag but your sniper class doesn't have grenades? Bring out your assault class! If there is no class system, you could have a sniper running around with a sniper rifle, grenade launcher, grenades, etc. etc., making the game extremely unbalanced and unfair.
Just my 2 cents.