I disagree! For me at least, there's a real psychological difference between making small, incremental progress and *no* progress, and having to manually set the exp all the time forces me to always be aware of it. It makes every little action feel kind of hollow and pointless, because I *know* I'm not making any progress towards leveling up.
Setting my exp gain to be lower, once, and then never thinking about it again is vastly different from having to constantly switch exp on and off again.