But were you previously spending £40/$50 a year on PSN? If not they are probably getting more out of you now than before. Especially if you also buy more games in future because of PS+ discounts.
I think PS+ works in multiple ways:
The hardcore Sony fans will probably have a lot of these free games already but they subscribe for the cloud saves, betas etc. They don't stop buying new games because these guys want to be playing on day one. They might end up buying even more through PS+ because of the limited time discounts. They would have been paying a lot of money to Sony anyway so the idea is not necessarily to get more from them but to keep them happy and deter them from finding other things to do with their time.
The occasional PS3 players may subscribe because it seems like a steal but they are now hooked into paying Sony $50 a year. Some of these will actually be big gamers who spend more time on 360 because it's where their online friends are or whatever. If Sony gets these guys to spend a greater proportion of their gaming time on PS3, and ideally bring some of their friends with them, it's a definite win. Some of these people might also now be questioning the value of their XBL sub. I don't think the idea of PS+ being a deliberate spoiler tactic for XBL should be ignored. This is especially important as we approach the transition to next gen.
Some of the very casual gamers or those on a tight budget would have been buying games primarily from the used market. If Sony can get $50 a year from these guys it is almost pure profit.
Another interesting aspect for me is that it gets people used to digital distribution of huge multi-gigabyte games. This might be a consideration for next gen.
Of course there might be some customers who manage to use PS+ in way that means they are spending less money on PS3 now than before. But I think the overall picture could be pretty positive.
It's supposed to be the best one, right?