Everyone seems to agree that this is a good idea but I'll go ahead and put my 2 cents in as well.
YES
Having capped leagues is vital to the NPE. When you start off playing, learning the game, it is critical to play/learn against similarly constructed decks. Being used as a 'stepping-stone' by a OP account is not constructive nor is it a positive experience for anyone just starting the game.
As for League Leader Boards- The less Rewards offered for those, the better. In my opinion SPS, DEC, and Glint would all be too valuable as LB Rewards. There are a few alternatives which could split the difference between, say, 1K Glint and no Reward:
RE: Community Poll - Reinstate the Card Level Limits by League