Some thoughts about Planeswalkers.
The "fixed deck only" play is more or less set to make online matches as even as possible, but I'm guessing it's also to keep the game as limited as possible for a few reasons.
I don't know if that's to sell as many expansion packs as possible once players get bored of the same pre-built decks, or if Wizards of the Coast want video game players to start playing the physical card game which is designed in a way that it essentially has an unlimited spending cap for card decks, packs and expansion sets which have levels of rarity in addition to being randomly packed. Both, I'd imagine.
The fact that buying the game gives you a voucher for an exclusive physical card that you have to pick up at a M:TG retail outlet pretty much supports this: the card's only useful if you play the physical card game.
I also noticed that the expansion packs are already included in the base game's installation: buying the DLC only gives you a 100KB key to unlock the extra content. So basically it's already in the basic $10 game; you just have to pay more to access it.
I'm guessing that any future expansion sets will simply allow players to unlock more decks as opposed to actually "buying" new decks and downloading more cards.
And this is the real reason for the DLC/expansion set structuring and fixed deck set up of the game: you wouldn't have to buy new decks (or more accurately, pay to unlock them) as you could simply build new decks using the cards you already had access to. Naturally, they want to sell you more prebuilt decks rather than let you build your own.
That being said, the game still plays just like the physical card game without having to spend the hundreds on cards it would take to build all of the decks in the video game. Unfortunately, it takes out the most important aspect of M:TG, which is deck building and fine tuning.
But considering the base game and three current expansion sets cost $25 and allow you to play people online, it's still a lot cheaper than buying M:TG cards and playing people locally. As any player of the card game will tell you, $25 buys you next to nothing in physical cards. A mixed starter deck (that is unplayable since they're randomly packed) and about 4 booster packs (randomly packed cards). You'd be lucky to get one half decent playable deck out of that.
You could order cards online and buy 20 mountains, 4 shock, 4 incinerate or whatever burn, and 32 goblins and probably end up spending less :) You would also win a fair amount of games if you get the mana curve right.