While outside of the period, the illustrations in the Beauchamp Pageant (c. 1480) generally have part-armoured men with bare arms if mounted and bare legs on foot, alongside the fully-armoured guys.
The Perry explanation seems backwards too; fully enclosed thighs would be redundant when mounted and extremely uncomfortable, chafing and even possibly cutting-off circulation to boot. The other odd bits that developed for fighting on foot would also be uncomfortable and restrictive when in the saddle too; especially the variations of 'ass-armour' that developed (the technical term I believe).
I seem to recall from somewhere that Mr Average either swung for the head and neck, struck down for the head and shoulders, or thrust at the torso, and indeed men appear to have gone for a helmet, torso defence and a collar/shoulder 'standard' in that order. The limbs seem secondary and the preference for arms or legs when all could not be protected, seems dictated by battlefield role. Hands and wrists also seem to have been protected before the rest of the arm too.