Purely digital distribution is nice for a lot of reasons, but the one drawback is huge.
In 2005, I purchased the album "With Teeth" from Napster for $11. Cheaper than buying the CD, but it was great in that 1) it allowed me to make an impulse purchase, and 2) I was able to get the music on my workstation (where I listen to about 95% of music). The drawback was that it was DRM encrypted and that now, 3 years later on a different PC, Napster has "forgotten" that I purchased the album and thus I no longer have access to the tunes -- unlike purchasing a CD, I merely ended up renting the music for a couple of years.
Same deal with Steam. It's an awesome content distribution system that has grown a huge library of available games and has gotten to the point now where you almost have to wonder if Valve is making more money as an online vendor than they did as a game developer. I purchased Half-Life 2 off of Steam the night it was released and was able to play the game within less time than if I'd jumped in the truck and driven to Wal-Mart for some 2am shopping.
The only problem is that now, 3 years later, I have an mild interest in replaying part of the game but am out of luck because I forgot the clever username/password combo, and don't remember what anti-spam address I gave them during registration time. From a self-help perspective, there's no way to recover the account. I don't have a CD and thus no CD key to use. Fortunately, the fact that I thought about all this before consciously making the decision to purchase (I guess "rent" is a better word now) the game and that this was the only game on the old Steam account have made it fairly painless to walk away from.
But it does give me a moment's pause when considering spending $20 on other casual games they have for sale there.