Roland's lever IS variable, you just have to be a bit subtle with your pressure. It's definitely NOT all on or off, like some seem to think.

Leslie on/off? Got an FC-7 assignment for that, why would I want to take my hands off the keyboard?

And voice programming..? Well in a WS or synth, yes, you've got time to hang out down at the left end, but arranger play doesn't give you that option, often (as I keep trying to say!). Having a slider in the middle allows the RH as well as the LH to adjust this kind of thing, and the RH may have the time more often than the left. It's not perfect, but it allows you to vary a variety of parameters rather than the ONE mapped to modulation wheel (it's called that for a reason!)

For synth playing, and non-arranger use, yes, I really like the two wheel system, but in arranger mode, it's a bit restrictive, IMO...