There were many reasons for this decision, the main reason was that there were a few noobs who ran ModMii.exe from within the .zip archive instead of extracting all the files first (this led to problems where ModMii couldn't always find some of its supporting files). The installer ensures that ppl will never make that mistake again.
Another great thing about the installer is that it does not require the user have an archive manager (ie. winzip or winrar) installed on their PC in order to extract ModMii from the zip.
And the last great feature to the installer is that it is capable of installing optional shortcuts to the start menu and to the desktop, which some people find useful (it's also another way to really showcase the amazing icon you made!)
There are a couple more good reasons that are really only a benefit to me... for example, I don't have to keep updating the main thread with new download links with each release. Also, there are may sites that are uploading mirrors of ModMii even though I've specifically requested people don't do that. Frankly, I don't care if people mirror the installer, since the installer will still grab the most recent release from my google code page (I like being able to track how many times ModMii has been downloaded), but I don't want people mirroring the actual program. So hopefully the ModMii Installer will put an end to the many ModMii mirrors out there.
Lastly, if you, or anyone else, still want to be able to download ModMii using the old method (ie. in it's zip form) those links are still active and can be found here:
http://code.google.com/p/modmii/downloads/list