Leather Cuirass Lorica Musculata, I used to think no way but

[Paullus Scipio]

Oddly enough, I have worn a suit of armour like Tarbicus' Gothic plate illustration for most of a day and found it relatively comfortable ( especially with the sallett tucked under arm! )...it belonged to a friend, so was not made for me and was not a good fit therefore, but nevertheless was eminently wearable. I have also been a soldier and can endorse Dane's comments.
'Uncomfortable' gear, like that Highlander's parade outfit, is usually inflicted on soldiers by those in Authority and quickly succumbs in the field to practicality...for example, British redcoats of the Napoleonic era were supposed to wear leather stocks to keep their heads facing front at all times (BTW so were U.S. Marines and British Marines of the same era - hence 'Leathernecks' and 'Bootnecks' as nicknames)...but the numerous peninsular diaries tell us how this and much else 'official' gear was junked in the field, to the point where units were unrecognisable......and take a look at photos of modern U.S. or British or Australian soldiers in Iraq/Afghanistan...they are supposedly'uniformly equipped' but no two individuals are alike......

As to protective efficiency, no doubt every mediaeval soldier wanted that Gothic suit in battle, but it was not practical to march day in and day out in one, so even if he captured one, he sold it and settled for a Jack etc....or Alexander's veterans marching to India and back over 11 years - but only fighting four or so major field battles in that time plus a few sieges, and various minor battles and skirmishes.....even with slaves and pack animals, you'd want to trade off protection ( on the few occasions you are going to need it) against practicality that encourages 'light'......