Just to clarify things, you can also buy or craft the hats. 4 Refined Metal + A Polycount pack item for the class = Polycount hat for the class.
Plus, *none* of the item sets provide a true gameplay advantage. Sniper's Sydney Sleeper is basically junk, making the set basically junk. The Degreaser is notably less powerful, and the set bonus requires you to use the Powerjack, which goes better with the regular flamethrower. The Scout has to use Mad Milk as a secondary, which reduces his offensive power significantly. The Pistol is great for gunning down unattended sentries, which Mad Milk can not do. Soldier has to give up his Shotgun AND use the Black Box, both of which severely hamper his offensive ability as well, all he gets out of the set bonus is an modest increase in sentry breaking (which he could easily use the Direct Hit for instead). The Spy has to use YEW, which means he can not disguise at will anymore, and the set "bonus" isn't really much of a bonus, since it also has a drawback. Most TF2 games are loud enough to cover up the Invis Watch/Cloak & Dagger's decloak (and most players suck anyway), and using YEW + The Dead Ringer means you can not disguise OR cloak at will - a massive weakness that requires either immense skill or blind, deaf opponents without hands to overcome.
So, hey, once again, a mountain is seen where an ant hill stands.
Of course this doesn't stop me from SAYING that purchased items are more powerful.