Some people have strong reservations against .umod installers (it doesn't really matter here whether they are justified or not), and some people just want to have complete control over the installation process (like, making sure that nothing is overwritten that makes weird strings appear in dedicated server windows---*cough*), or are plainly curious about the installer's content before they install.
So, I really suggest that if you publish something in .umod format, always provide a plain .zip version as well. (If the .zip contains subdirectory information for the files it contains, it's even almost as comfortable as a .umod installer.)
I also suggest you create some sort of batch file or script that streamlines the process of creating your distribution packages: Creating the .umod file, packing it (along with its readme file) in a .zip archive, and creating a separate .zip file with the individual files for people who don't like .umod installers.