A major disadvantage of a non-Admin account is that you can not install anything or authenticate anything. You have no Admin authority. To install any OS update or application you should be logged in as Admin. Normally, this would involve logging out as non-Admin and logging in as Admin.
With Mac OS X 10.3 and later, an alternative method would be to FUS (Fast User Switch) into your Admin account. Do the job and FUS right back out. When you return to your non-Admin account, everything is there as you left it.
Memory, or rather the lack of it can be an issue depending on how may applications are open in each environment.Word can be opened in each environment, but its memory use doubles with two user accounts and quadruples with 4.
You can not make changes to user accounts if they are logged in.
If you try to shut the Mac down or restart it while other user accounts are logged in, a dialog box tells you “There are currently logged in users who may lose unsaved changes if you shut down this computer.” You now are asked to type in your Admin password to demonstrate you have the authority. So, if you are doing an OS update, might as well quit all applications in the non-Admin account as you will need to re-boot upon install of the update.
All application installs should be done from the Admin account, so that all applications get installed in the root Applications folder and not the Applications folder in your user directory.
There are certain limitations where your Home directories must be.