Depends on how you are leveling your character. If you're trying for well-rounded development, then you need to get heavy armor points, & using a heavy armor shield when you're working on your block is a good way to do it. You don't have to pick the heavy armor perks, you can just gain experience with the skill, which will improve your performance while wearing it.
Say your character is at level 55, & he's maxed out at light armor, so leveling up is hard to do. So you switch to wearing heavy armor, which is still at level 15. All your enemies have leveled up, & you are going to be at a serious disadvantage when you fight them.
I tend to go for well-rounded characters, myself.