Light armour is always fun to use but the difference between heirlooming it and not is a great difference. I run in Reinforced Leather Jerkin which has 27 body armour & Lordly Mail Gauntlets, providing another 13, hitting me at 40 body armour.
Not quite into the top medium armour but it's a nice amount, countered obviously by the fact that I don't have a decent helmet, Sturdy Straw Hat with 3 head armour is pretty useless, although I did survive a headshot from a bow the other day, but that's besides the point, bloody peasant archers.
Reducing the weight of the armour each heirloom would eventually mean that archers could outrun Sumpter Horses, obviously a slight exaggeration but there will be a point where my 3.5 weight becomes 2 on the Jerkin, and since I see a massive difference in 1 weight, 2 and 7 athletics (33 with 8 ath) would mean that I would near enough out run everything.
Although I don't think there is anything wrong with light armour, I would more appretiate a slight increase in the amount of armour they get, even if it's one so that on the final level, you gain 3 armour instead of 2, something like that, not that it effects me too much because if I am hit, it'll likely kill me anyways with the way Arbalests/cav are the two things that kill me more than often.