Northeastwrecks once bubbled...
First, if I don't, I'll need to carry weight. Extra air is more useful than extra lead. Second, its good practice to dive in the same gear that I use for more challenging dives so that I can practice my skills.
The air vs. weight issue is why I tend to use steel 120's in my recreational set-up instead of AL 80's.
I also believe in the second issue. Buoyancy control is more difficult at shallower depths anyways. If I can practice there, I do far better at depth. Also, certain skills can cause buoyancy issues (for instance, shooting a lift bag). I would rather do my practice at 30' when I don't need that bag and perfect the skill than screw it up at 120' with a deco obligation when I need the upline. If doing it for that reason, I feel that I should be in the rig where I am most likely to have to do it. That would be my doubles.