I would think that a high agility means that they better execute learnt movements. And if the choice were Ag as a Development Stat versus In, I'd take the latter every time.
Also, I don't think that 'all stats as dev stats' is the answer. The answer is, at least, to drop Ag as a dev stat, maybe Co (I'd drop it), and add In.
Also, of course, get rid of Em (which is a crock) and move the Mentalism base stat to SD.
Along with the 'dev points from stat bonus' option, this would suit pretty much all of my concerns with stats (increase importance of mental stats, nuke Em, get Ag out of being a dev stat and screw the damn elves as an extra bonus).