Considering what peripherals, graphics cards, antivirus software and all the other modern day stuff that gets downloaded into your PC and run as background processes, you are looking to have eaten up in the region of 1.2 to 1.5GB just by booting your OS and doing a few daily things like checking email. Considering there are still a lot of Vista 32bit gamers out there, they have a Ram ceiling of about 3.5GB. So a lot of gamers will be running near the edge of having 2GB of 'free' memory left for gaming. Thus a 100MB exe running in the background can be a significant overhead. Steam is also trying up processor resources (which is normally running at 100% when I'm playing Oblivion on highest settings) and can slow down parts of the game by needing to be swapped in and out of memory.
That is now.
In 9 months I guarantee that your repeatedly evolving graphics drivers, patched OS, updated antivirus software and all those other background processes will have grown significantly - most developers assuming that they can hog all the system resources they need (hey my program is more important than anyone else's) or that everyone has already upgraded to Win7 (still on Vista, are you a Luddite?). Steam will be foremost amongst them. Therefore in the not too distant future, 32bit Vista even with '4'GB and all the hardware to put a PS to shame, will start to suffer problems trying to find enough gaming RAM without having to constantly page/cache memory.
Now not everybody has the technical skill or knowledge to purge their system, running it clean. Some need a lot of that other 3rd party extraneous software for their jobs or hobbies. Others lack the patience to do clean sweep and reboot every time they run a game. Still more have currently powerful gaming rigs which they don't want to upgrade to 64bit. So for them it'll start to make a difference. At the moment it might be a mere handful of FPS, but those can become vital in preventing lagging depending on the on-screen action or numbers of characters in your cell. In the future it'll just get worse and Steam will be a significant portion of that responsibility.
So don't say 100MB isn't much. It does has an effect. If nothing else, unless Skyrim plugs all the memory leaks Oblivion had, it will shorten the play time before the game CTD.
Two things:
"Now not everybody has the technical skill or knowledge to purge their system, running it clean"
Well, they should. True that I got W7 x64 down to http://img827.imageshack.us/img827/9383/w7x64withmseandsteam.jpg with MSE and Steam running. But I actually do normally run with Asus Fast Boot and Tune Up Utillities and I'm still well inside the 30s with those.
Link for people who need to learn to optimize their OS [XP, Vista or W7]:
http://www.blackviper.com/
Secondly, 100MB doesn't really matter even where having 6GB vs 3GB is concerned. 100MB isn't going to do enough for it to matter because if you're cutting things that close, then your already over the cliff.
-If you only have x86 [aka: a 32bit OS], you're limited to 2GB. If you have 3 or 4GB + 1GB for the GPU, then you only have 3GB total anyway. Your game has all of the 2GB it is going to get and 1GB is twiddling it's thumbs with little to do.
-If you have 4GB on x64 then you should be close to or as good as having 6GB. It always depends on the game of course, but that extra gig in the jump from 3 to 4 is more important than the further addition of 2GB more. Warhammer Online gets close to 4GB total, but there is some headroom on having 4GB still for the most part, a lot in most cases.
http://www.tomshardware.com/news/corsair-triple-channel-ddr3,6614.html
http://www.bit-tech.net/hardware/memory/2008/07/08/is-more-memory-better/5