Critical hits add a spice of much-needed randomness to tf2 to keep it from getting stale. It means you generally need to stay on your toes, and can't ever be 'completely sure' of your odds in any given situation. TF2 is generally a very chaotic and funny game, with a light, comedic tone and style, and the added randomness and intentional unfairness of random critical hits fits that. It makes fights less predictable, gives lighter classes a better chance in melee combat, and, of course, lets F2Ps score a kill against a 4000+ hour golden pan sweatlord with a lucky crocket in right place at the right time. Oh, and reflecting a crocket then schadenfreuding afterwards is a cathartic sensation no other game can really deliver. If you don't like randomness and chaos then TF2 probably isn't the game for you and you should play something else.