Stupid but essential question

[eastbay]

I have read countless times that traders can't beat the market. You're better off buying an index and holding, because this is the best you can do. Every trade is simply throwing money away on commission from this baseline return.

True or false?

If true, why do investment banks employ traders? Sure they should know better if it is not possible to "win"?

If false, why is this claim made so consistently?

eastbay