I had always thought the "No Steam, No Buy" thing was a joke like the PC Master Race thing. Then I realized some people were really like that. Why you would game on the greatest platform in existence and then foolishly lock yourself onto essentially one storefront and diminish its greatness is beyond me...
If playtime tracking is a big deal, there's Raptr or Xfire (though I haven't used Xfire since it got bought out twice and started sucking). For the "whole library is there"...I understand the convenience, especially if you like to reinstall your OS frequently, but you can also add non-steam games so that you can use the chat/screenshot features of it.
The only thing it'd be missing at that point is the achievements on steam.
The big problem with "my whole library is there" is that also makes it a single point of failure. Especially when you aren't able to reinstall your purchased games without authentication.
As an example, what happens if Facebook swooped in with a pile of cash, buys Steam and then decides to rebrand it as a service for Occulus and mobile games while quietly leaving PC to die on the vine?
Can't spend your life worrying about what ifs, friend. I mean, what if you get hit by a bus tomorrow, or suffer a massive heart attack? No doubt as you're laying there dying, your final thoughts would include how you wish you hadn't wasted so much of your life gaming and browsing the internet. Does that mean you're going to sell your computer and dedicate the rest of your life to making a difference and bettering yourself and humanity as a whole? Hell no; you're going to sit there on your ass and play video games.