synfresh: In 11+ years of using Steam I have never ever been denied access to any game I have ever purchased on there.
To counter: with the first - and only - title I've ever purchased that required Steam (and I bought a brick-and-mortar package with honest-to-goodness installation files on an actual disk, not just a Steam key), Steam denied me access to a game simply because I didn't acquiesce to the Client's EULA AFTER the game was installed. For a game I didn't buy there, for which I didn't need any access from Steam for the installation, for an off-line game that is now here DRM-free. Thank goodness the developer (TaleWorlds) offered up a workaround.
YMMV.
I'm sure Steam works great so long as you do things their way. But when I refused installation of an unnecessary client attached to the game, Steam refused to let me play a game that I bought elsewhere. That was enough of a warning.
Now, I don't tell people not to buy from Steam or buy Steam keys elsewhere. But they should do so with open eyes. If the customer is okay with how the client operates, then more power to 'em. Most won't care. Some do.