OK so I did some reading and I think I understand this more how it works..
You have 2 options when Half-Life 2 is released...
1) You can buy the game in the store (50$) and play online for free as long as you want. But when CS2 comes out, in order to play you will have to go to the store and buy that game as well (40$) and same goes for TF2 (40$)... But you can play online for free..
OR
2) You can buy a subscription to Steam for 9.95$ a month and download these games at this price. Meaning you wont have to pay the 50$ HL2 will cost in the store you will just have a massive download. Then When CS2 and TF2 is released you will be able to download them at no extra cost. Also I would assume this would go for all other games they come out with.
---- I think this is how they are doing it. From my interpretation it was, but I may be mistaken. So really I think both deals are fairly good, I think it just depends on how many different type of games you are planning on playing.