In TES, one of the biggest advantages of Light Armor is that you can carry more loot. Unless you travel with practically nothing with weight, you're not going to outrun many of the opponents, so increased speed isn't all that much of a benefit. Agility is not affected by armor in TES, so there's no reason why your sneaking thief or assassin needs to stick with less protection.
The entire Light-Heavy armor divide is completely artificial. I would much prefer a system where every piece has its inherent good and bad points, so it makes sense to wear particular types of armor for specific purposes. A thief should have drastically different armor needs than a fighter, and an assassin should have some differences in needs from either of those. Balancing protection, weight, durability, repairability, cost, and availability, in order to create a complete spectrum of types, seems like a much more reasonably approach than having a "rat race" where you have to keep getting the latest "simply better" type for your new level, and having two or three distinct "lines" of armor to progress through, with no real distinction at the end when Perks cancel all of the disadvantages.