Some of my better tournament memories have involved receiving an absolute thrashing - you learn more from them than yet another close loss that you blame on poor dice rolls.
yup, one of my fondest warhammer memories was getting my ass handed to me buy a couple of snipers and a rhino full of tactical marines. it helped break me of my old mono-death company ways.
also in regards to the whole tyranids don't have good anti tank defenses, what about the heavy support slot? old one eye and the other monstrous creatures they got milling around down there can pack a serious punch.