Re: 4x weak - to battle or to switch?
Of course it's dangerous, but it also has advantages. Plus, you bait the trap for a great countering move when they knock you down to 1 HP.
Besides, I love Aggron too much to care for the 4x weakness to Ground and Fighting. Ground can be circumvented by Magnet Rise ;D
Bleh. I'm not a competitive battler, though.