The chances of Steam shutting down and irreversibly suspending my access to my entire games library in perpetuity is statistically infinitesimally less than me having all my games stored on disc and my house burning down.
That was just an example of any number of "acts of god" that could deny me continued access to my games. What about hard drive failure? Effectively I have to pay every year just to ensure I have working hard drives on a continued basis, more money more often if I want backups. Even then, I've had hard drives fail and I've lost my data. I've had burned CDs melt in the sun and no longer work or otherwise experience data corruption, and new discs and disc drives also cost money. Off-site backup mechanisms cost money via an Internet connection and subscription fees, more if you have a data cap that you exceed.
In other words, data is impermanent, but I think any backup solution Valve has is probably better than what I reasonably want to pay for, and all for the price of free - or heck, you could argue I am
saving money for this feature because I have access to Steam sales and can get most games cheaper on Steam than anywhere else. I don't consider this a negative.