[100MB with the UI isn't much. People should have 4GB [...if not more] in their rigs if that rig is meant for gaming.
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.