I believe practicing gymnastics has great carry over to Parkour. Gymnastics is a more rigid system but it teaches body awareness, control, and strength that few other non-PK activities can. After you've done some gymnastics training you can see what has the most potential of helping your PK training and then go from there. If given the chance I say go for it, you're more likely to regret not doing it.
Warning, text wall! But it's good, I spent a long time refining it, so please read!
As for the discussion going on between DaveS, Steven, NOS, and others, here's my insight. It seems to me like the disagreement is boiling down to this. Either you believe Parkour is mainly a physical pursuit and the spiritual/mental/etc improvement is a by-product of this (a la many sports in which character, respect, and other qualities are expected of great champions). OR you believe Parkour aims for all of the above from the outset, the physical/mental/spiritual improvements being one and the same and thus inextricably linked.
I can see the pros and cons in both views. Seeing Parkour as largely physical allows for greater enhancement of the skills and movements used in typical training, this being achieved through the wealth of knowledge about exercise science. At the same time, this physical focus does not guarantee that the practitioner will adopt the other qualities we seek, such as a football star becoming arrogant in his abilities. An integrated approach can better counteract this human tendency by making it essential to training from the outset. At the same time, failing to focus harder on certain areas leads to dogma and weaker training overall, such as martial artists performing strict forms that have little use in a fight. The practitioner is more likely to stay balanced at the cost of peak ability.
It appears to be a catch-22. Reach for the highest levels of movement and you risk losing sight of the other qualities Parkour seeks. Focus too greatly on general improvement and you may plateau, never moving any further in one direction or the other.
I have no contact with the founders and early practitioners and thus can't speak for them. But I'll bet they never dreamed of the movement possibilities that are practically common place these days. And I feel the only way we hit these great heights was by viewing Parkour in a narrower light. They saw Parkour as a more balanced system, whereas quite a few practitioners now see Parkour as the movement itself.
In the end it becomes a personal choice, of seeking the highest limits or the greatest balance.
EDIT: And make no mistake, the idea of Parkour is evolving and changing daily. As David Belle said while being interviewed by Foucan
, "Yes, I think young people found what they wanted, anyway, young folks always adapt to anything." He agrees they made a mistake in putting out the first demos that helped create the physical focus of PK we see today, but admits he hadn't finalized what Parkour was, what his father had taught him, when he first released tapes.
"And you think, well, if they get their kicks out of that, you can't really say 'No, you can't do this!' You have to let them express themselves. But we should also be entitled to finish what we started, even if people don't adhere, it doesn't matter, since we never did things to please or bring crowds, so we just stay ourselves." David and others let the cat out of the bag too soon, and now Parkour has evolved according to the masses. If you watch the whole interview you can see that Parkour evolved within David's mind as well. This seems key to the idea that Parkour's purpose or reason can morph with the personality of the practitioner.