There's only two especially strong arguments I've seen against Steam here or elsewhere. One of them's the fact that, should Valve go out of business, you'll lose access to all the games they're hosting... but that one's not especially compelling. It's a bit common for people to think that the current markets work so that things just happen suddenly and without any warning, but that's simply not true. The example people usually use is the recent recession, but the fact is that economists saw that coming for an extremely long time - it didn't really sneak up on any of the industries struck by it and it certainly didn't sneak up on anyone monitoring those industries, it just happens to be something where a lot of people, politicians, and companies ignored the long-term consequences of their actions for the sake of short-term gain. Basically, there were warning signs on that one visible from miles away.
So what about Steam? Well, look at Steam itself. Valve's used their own extremely popular titles to entrench themselves so deeply into the downloadable content market that even some larger publishers who've attempted to set themselves up afterwards can't hope to compete (EA has their own online store, but typically releases their titles on Steam anyways simply because Steam is where most of their potential customers are). They've started introducing features like Steamworks that make it so that competing services can't sell their games without packaging Steam with those games (which they won't do), making it pretty much impossible for services like Impulse or Direct2Drive to actually compete with Steam. And with the market and consumer base that Steam has now, they have so much momentum that even an exceptionally well-funded and well-developed startup could never hope to compete with them.
And even putting all of that aside, look at the overall market. Virtually every media market is expected to shift toward digital downloads. Netflix is killing Blockbuster, services like Hulu are eating away at the cable industry, newspapers are struggling to compete with their online ad-supported counterparts, devices like the iPad and the Kindle are becoming increasingly popular, and more and more video games are selling through digital distribution services,
especially on Steam. This is a trend that's constantly picking up and that has absolutely no competition and no reason to change, so there's absolutely no indication here that Steam's going to do anything but get bigger for quite some time.
All that said, does that mean Steam's going to be here forever? No. Of course not. But it does mean that the service is going to be around for a very long time. Even if we run with a rather questionably short time frame for its remaining lifespan of about 10-15 years (considering how long game development takes and how sluggishly the games industry tends to move)... well, 10-15 years is an extremely long time in terms of technology. By the time that's passed, Steam going out of business won't matter - getting the games that had relied on it through other means will be so trivially easy that its disappearance would affect virtually nothing aside from some mild level of convenience.
As for the other good argument... this:
My problem is pretty simple: I have a high end computer running my music studio in the basemant that I will never connect to the internet. I run Morrowind, Oblivion, and Fallout 3 down there. My internet computer upstairs cannot even run Oblivion. I download mods and transfer them downstairs with an external drive and I'm perfectly happy with that setup. I was able to set up Operation:Anchorage and The Pitt using Windows Live by that process. I cannot set up Fallout New Vegas because of it requiring authentication via Steam. I'm unhappy about that. If I want to buy Skyrim, and I certainly do, I will need to buy a new computer and run it upstairs or run an internet cable downstairs and run it down there on a new computer. It would be nice to be able to run it on my existing high end computer without needing to ever connect it to the internet. Other game systems like XBox or PS3 are out of the question, because I don't play enough different games to justify the cost.
You ask why I don't have my music computer set up to run the internet? Because it's lean and mean and I want to keep it that way. I know myself and I'd just get it all clogged up with a bunch of junk I don't want on it. I'm stubborn like that.
SO, I GUESS I NEED A NEW COMPUTER.
Steam? I know little-next-to-nothing about it, but I guess I'd use it if I had to.
And this is the main problem I have with Steam. Skyrim's almost definitely going to be a single player game. It's also almost definitely going to use Steamworks. Steamworks comes with some nice perks (Steam Cloud support being the best one), but it also means that retail copies of the game are going to require an internet connection for installation. It may not be common, but (believe it or not) some people still don't have high-speed internet at all, and a few more don't have it connected to their gaming machines. They're basically stiffed by this sort of thing, and unfortunately Steamworks isn't something that can really be handled in an optional sense or separately from the Steam authentication - if a game's going to use it, it's going to need to connect with Steam when it installs.
For someone like me this all works out pretty well - I always have an internet connection to my main machine, and Steamworks means Steam Cloud access, better Steam Community support, and automatic patching - but it's still a pretty bad deal for anyone who doesn't have a net connection. Not that that'll make much difference - it's a business decision for Bethesda, and unfortunately the group of people with no internet access or no broadband who are interested in their games is so small as to be insignificant to them.