John, the thing is that registration buttons are great for gross sound changes, but one of the things that many real organ players do is 'shade' the sound in realtime, as you go along. It's not a case of just completely re-doing the registration, but a constant (or near constant!) very slight adjustments to the drawbars. One phrase may get a little lost, you pull out the 2' a click or so, push it back in after the phrase, you might take the 16' out a click or two for a quick line, and push it back in a bar or two later. All these minor tweaks are too numerous to be stored as preset registrations, they really ARE something you tend to tweak as you go along.
Got a guitarist one night with a scratchier sound than your usual? Shave a hair off you high end to not clash... Got a duller room than usual? Spice it up with a bit more top three d'bars... Band getting muddy? Push back the 16' a hair.
Registrations only get you so much control. The drawbar (or buttons, it really doesn't matter as long as you CAN adjust in realtime) is the final step in making something perfect (for right now). They are STILL on modern organ clones. If they were of so little use, seeing as how you can store as many presets as you like, they would have been dropped long ago (they add considerably to the cost of a clone).
For most REAL organists, the thought of a B3 without drawbars (or ANY way to adjust in realtime) is unpleasant
In an arranger context, you have to decide for yourself whether these subtle tweaks are worth it or not. I use my arranger with live bands as well as duo and solo work. Especially in the band situation, the drawbars are a MUST. In an OMB situation, where you have total control around you, maybe not quite so much, but once you get used to shading your registrations, it's hard to stop!