I think that the concept of the MWL and the sort of functional errata we have (it looks like we have a theme of only functionally errata-ing deckbuilding aspects of cards, not gameplay aspects) are both somewhat poorly targetted and not quite sure what they're trying to achieve.
The MWL is supposed to deal with cards that are just a bit too effective, but does nothing to decrease the efficiency of those cards. It only means you can play slightly fewer overpowered cards in your deck! In terms of the actual use of the MWL, the cards going onto the MWL (in both batches so far) seem very clearly targetted at cards that "combo" in popular decks. In other words it seems that it is being used as an attempt to take particular deck types down a notch, not individual cards.
Errata to deckbuilding aspects of individual cards (one per deck, unique as a pseudo-deckbuilding element) again don't deal with broken cards. They just affect decks.
Trying to target decks through individual cards in this particular way (rather than actually fixing problem cards) is a tricky proposition: decks are a constantly moving target, knowledge of the knock-on effects is always sketchy and it always causes a lot of unnecessary collateral damage, reducing the possible variety of deckbuilding.
The approach taken does mean that there is slightly less pressure for card reprints (which I presume is the reasoning), but at the cost of being a poorer fix in game design terms.