Lack of foresight.
"BULKY MON GOOD DEFENDER, PUT BULKY MON IN GYM".
I used to always use a Gardevoir as my main defender (until I managed to get DG on her, at which time she joined my Machamp solo corps) just to deter Machamp spammers. Heck, at one point I managed to hold onto one of three EX-elligible gyms in my town for like a week just because the members on my team were genuinely smart on what they put in: IIRC we had Blissey, Gardevoir, Snorlax, Dragonite, Donphan, and Tyranitar No one species could sweep the gym, and with just a bit lf gold-raz play, I'm fairly sure that we all scored gold for the gym.