WOW, now here's an interesting thread!
This subject seems to really get organists going, and opinions tend to be strong on this. I'll chime in with my own opinions and experience on this issue.
My own opinion is that piano is not at all necessary, and can be counterproductive if the student really wants to play the organ and not the piano. Why do I say this? Because that's exactly how I was. Over the years, I got to really like the piano, but I never took a piano lesson until I was in college. I started on the organ in a "preparatory" program at Susquehanna University for local music students (not university students) when I was 13. The teachers were themselves university music students, and my teacher had no idea what to do with this rebellious upstart. She asked her professor, James Boeringer, for advice and he told her to just teach me technique at the organ. I even played some of Bartok's Mikrokosmos on the organ! But the point is that it worked fine for me, and if I had been forced to play the piano at that time I don't know what I would have done.
I do feel that for technical reasons, the piano can be counterproductive to sensitive organ technique if one is not clearly taught the difference between the two. The major difference is that organ keys have the "pluck" near the top of the keystroke and the piano requires that one "strike" the keybed. To pull through a pluck and to strike a keybed are two completely different things. However, I have seen many, many "organists" play insensitively because they are striking the bottom of the keystroke. Especially if playing a tracker, one needs to sensitively PULL or PUSH through the pluck; and then there is another important factor that can be largely ignored in piano playing--one needs to CLOSE the pallets sensitively, too, which means controlling the speed of the keys' returns as well. It is a completely different technique. In my opinion there is much insensitive organ playing because of too much piano technique at the organ. Even on an electric or pneumatic action, the playing will be much more clean if one is aware of the point of contact and not striking the keybeds.
For this reason, Giovanni's mention of the harpsichord is a valid one, to be sure. Nowadays I practice a lot on the harpsichord because it also has a pluck near the top of the keystroke, much more similar to the organ than the piano. The clavichord also has distinct advantages. Anyone who has played a well regulated one will immediately note the shallow touch. Interestingly, the shallow keystroke is just about the same as the distance to the pluck on an organ or harpsichord. So while one does need to "bed" the keys on a clavichord, this still involves a very shallow touch. One develops a very sensitive awareness of this area where the pluck occurs on organs and harpsichords.
In my opinion, these major differences make careful instruction necessary when moving from the piano to the organ, and it is often done unsuccessfully because the teacher him/herself is not aware or does not clearly articulate the differences. James Boeringer stressed the importance of learning to play tracker instruments and I still remember him stating in his colorful way that piano technique does not work at the organ. He said you could either press the key carefully or hit it with a sledgehammer, makes no difference in the volume! And how many times have I seen "organists" beating the keys when carefully planned use of body weight would work much better, even on the heaviest tracker action!
I will not say piano lessons are bad, just that careful and thoughtful instruction and practice are necessary when making the switch. Non tracker actions are more forgiving, but then one never learns real organ technique, either.
Thomas Dressler