What are the advantages of Light Shields over Heavy Shields? I'm a Light Armor user but even with a much higher LA skill the heavier shields such as Steel, Ebony, etc still seem to offer more protection. Having full Light Armor and a Heavy Shield doesn't take away from my LA full set perk bonuses, nor do I seem to run slower with a Heavy shield equipped compared to a Light one. What's the advantage of a Light Shield then (apart from it not weighing anything with the appropriate LA perk)?

I believe you answered your own question in the last sentence.

Honestly is that it? That means it's essentially useless for any build to use Light Shields, since they don't gain any significant bonuses beyond saving about 5 pounds and most of them are terrible in aesthetics (Glass, Dragonscale) or practicality (Hide...). I don't know why a selection of Light Shields were put in if they were inferior to their Heavier counterparts even for LA users.

Just to complete the sets is why. Don't use them if you don't want to.

Fair enough I was hoping there was some special reason I could justify sticking with a Light Imperial Shield when the Heavy one is always gonna be better.

If you have light armor smithing you can improve the light ones more. it's kind of like asking why anyone would use light armor when heavy gives more protection...

If you're into smithing and you're wearing light armor, you probably won't have the heavy armor perks needed to effectively smith Ebony or Daedric shields. An upgraded Glass shield with perk is better than Ebony or Daedric without.

+1 To what Maktaka said and keep in mind that there are no light shields worth mentioning in game at the moment so you might want to train smithing if you decide to go light.