It's not clear what the payment processing details are, or which MMOs will be adding Steam support. On the one hand, it gives gamers a simple way to manage subscriptions and expands Steam's "one stop" gaming center credentials. The flip side, however, is that centralizing all of your game subscriptions into a single location could make your Steam data all the more valuable to hackers. Steam stores a certain amount of credit card information already, but would need to store complete data in order to automatically process payments on a monthly cycle.
On the other hand, Steam's subscription services could be a boon for small and independent developers who don't have the resources to create their own payment processing network. Presumably Steam contracts to serve as middle-man for the payments, though that's still a bit unclear. According to Steam, canceling and resubscribing to a game may or may not re-link your previous account with the renewed one.
Will reactivating my Subscription give me access to my old game history?As someone who has subscribed to various MMOs over the years, this could be a handy way to manage the entire process -- but the MMO model has waned in recent years as Free To Play has become more popular.
This depends on the game. In some cases, your game data will automatically be re-linked to your Steam account. If this has not happened, you will need to contact the publisher of the game and have them re-initiate your previous game account and link it to your Steam account.