I've spent years using whatever bag I happen to have and lashing it to my rear rack with bungies. Panniers are godsend, because they're so quick and easy to put on and take off.

I decided against my pannier today, and I strapped my eight-pound lock-and-chain to my rack. I was fine because I have practice, but it reminded me that it's just not worth it. I should have taken the pannier.