Hi Scott,
Can I please benefit from your experience? I'm still essentially new to organs, learning my basic skills and still just playing on one manual with one registration. Big things are coming, I'm sure.
You said you would not play that organ I pictured with the piano on the bottom and the three organ manuals above. Hypothetically, if someone offered a serious organist (who also plays a little piano on a digital keyboard) an organ with three manuals, but also gave him the choice of an organ like the one pictured ... why wouldn't an organist take the 3x organ plus 1x piano manuals over just three normal organ manuals without the piano?
I'm guessing that the piano keyboard on the bottom requires you to reach higher and forward than you would otherwise ... but other than that, I can't think of a disadvantage over that 3+1 over a regular three manual organ. (I'm assuming both hypothetical organs would have three organ manuals. One just adds the piano in addition to the others.)
I guess the real question is, is having to reach up and over a "piano manual" a hassle and does that mess up an advanced organist? Does an organist get so acclimated to having the lowest manual right in front of him that having to reach over a piano keyboard to play ruin the playing experience?