Apparently my "similar to the following" text in man PKGBUILD was not enough. :(
I'd be fine with a check added to makepkg, although I never wanted to get draconian about it (and I never meant for it to become machine-parseable, but it seems I am in the minority there).
I know, but now they are there, it is just too tempting to leave them alone...
I added a bug report so this does not get forgotten: http://bugs.archlinux.org/task/15125
Even if pacman never ends up doing something with the optdepends, standardizing them makes the output homogeneous. More importantly it enables others *cough* to write code to parse and handle them. Requiring that a user specify an optdep either exactly as deps are currently specified or followed by a colon and a reason is not draconian. It's just sane and practical.