This is because Steam has a lot of useful features (download from anywhere, community integration, achievements in some titles, automatic patching, easy mod support, free DLC for some games, etc.), and few if any of these are available to freeloaders; SecuROM on the other hand adds no value whatsoever for customers, while the freeloaders get the overall better experience.
Valve has been so clever with the design and feature set of Steam that many users don't even think of it as being DRM even though as you say it shares some typical characteristics and is very much like a rental.

I really don't think it's a rental... you can make as many DVD/CD backups as you want, install games on as many computers as you want, and if Steam ever dies (unlikely, shit sells like HOTCAKES on Steam) Valve said they will release a patch making games not rely on Steam for installation/activation.

The backups are worthless on their own; you must install Steam and connect to the activation server at least once in order to be able to switch into offline mode and make the backups playable. The same applies to retail releases that use Steam for their DRM (e.g. Saints Row 2). My internet has gone down from time to time in the past, but it's never really inconvenienced me; for Steam purchases and Steam-powered retail games that suddenly becomes a bigger problem.
While I expect they would stay true to their word if at all possible, things don't always work out the way the employees would like when it comes to big companies being bought out or changing business practises or whatever.
If GOG disappeared off the face of the earth tomorrow, I would be very, very sad, but I wouldn't be out of pocket at all because I still have all my games backed up, and I can install and play them as far into the future as Windows itself supports them. I simply can't say the same for Steam purchases.

Yeah, probably. I just don't think that Steam will ever go away, and that's a bummer. I do remember when Prey was released, and that some small digital distributor, Triton went out of business. So happened they sold Prey, and they had to send out hard copies (like, on discs) to angry customers. Not sure if that was what happened, but something like that.

The only problem is, I don't want their "user-centric" features. I don't want to chat with my friends in the middle of a game, why the hell would I want to? I'm playing a game, aren't I? I know, some people think that is cool, but I'm not into the whole "stay connected to the whole world at all times" thing. For me, Steam is nothing but a nuisance.

It's more than that. It's stuff like achievements, in-game overlay, easy install/removal of games, etc..

Also, defragging individual games, browsing the web in-game, downloading DLC, etc.
Wishbone doesn't like the community features because he is grumpy and has no friends. D:

Who cares?
Of what?
If you think it's easier to download 4 GB of data than popping a single disc in the drive, you must have a monster of an internet connection.

I don't quite see how that would work, but I'll buy it as being marginally useful.
Sorry, what?
In other words, stuff that would be just as easy to do if Steam was a website, rather than an application you were forced to run whenever you wanted to play a game.
No, it's just that when I want to play a game, I start up a game. If I want to hang out with my friends, I go do that. And if I want to do it online rather than IRL, there are plenty of services specifically for that.

I hate how every thread that even remotely involves Steam devolves into another debate over why Steam sucks/rocks.
Why can't we just bemoan the fact that Steam is stealing our money with its awesome deals without having another one of these arguments?
Get your own thread. :P

Does steam have forums in which you could mention how many games you have, and how good the deals are?
I have to say, I'm with Wishbone on this. If it was a GOG-like thing, I would be all over it.

in game overly: allows you to browse servers, the web, and chat with your steam friends
easier to download 4gb than install by discs: I do, 30mb dl, 10mb ul!
not wanting to talk to friends while gaming: that may be you, but I find I play more windowed games so I can chat in irc. I'd really like steam to include an irc client.
dlc: autoupdates are fun, but sometimes I want to see exploits in action rather than seeing them disappear before I even hear about them.

Until a patch has actually been written and placed in escrow with a third party such statements should be considered completely meaningless. Even if the individuals making those statements are being completely genuine, conditions that would necessitate the release of such a patch (bankruptcy, acquisition and restructuring, etc) would pretty much preclude the people who made the promise from being able to make good on it.

Perhaps they could demonstrate this patch if they started to use it to GOGgify a few games that have been out for a few years and are less likely to generate a piracy explosion. How much would eidos lose if Thief 3 was DRM free?

We might have eventually ask Square Enix on that one. ;)