Well, I use a little bit of a bounce at the beginning of the leaning-away 1 arm laterals, which allows me to use the 40's. I will note that my arm is straight, none of this bent at 90 degrees BS, and I really try to milk the pause at the top, before lowering as slowly as I can. There's no way I would use the 40's for strict seated laterals though.
I can understand trying to get your body used to a heavier weight, but only if you can honestly get something out of it (ie. actually working through the negative portion like I try to do). All in all, I don't think this particular exercise is really suited to worry about how much weight you lift though. That's probably why so many people never develop a great 'capped' look to their shoulders.