I think that PS+ was probably the safest way for Sony to implement a subscription service during the middle of the generation. Let's be honest, there would have been a tremendous amount of backlash if Sony suddenly decided to restrict online multiplayer to paid subscriptions.
Instead, Sony chose an optional service that rewards people for spending money, through free games and discounts. While it is probably not nearly as profitable as Xbox Live, I wouldn't be surprised if one of the reasons behind the service is to establish stronger brand loyalty among its user base (especially during the transition into the next generation). Much like how Gold subscribers are likely to stick with Microsoft in the next generation, I can see PS+ members more likely to stick with Sony.
Regardless, with Gaikai in the mix, we're probably going to see a dramatic evolution in Sony's subscription model. Back on topic, I can see Microsoft occassionally offering free games, but I don't think that they would market it as a main feature like Sony does with PS Plus.