Why install for "just me" as opposed to "everybody"?

"Well, I think the answer depends on how the application you're installing has been developed.

Does it correctly place all user-created data into appropriate folders under a user's profile, or is it designed to run universally (and store user-created data in a common folder structure such as all users)?

Does it rely on registry keys in a user's profile (HKCU) or does it rely on a common registry (e.g. HKLM).

Does it require admistrative permissions to execute? (might be a good reason for the ""Just for me"" option, if non-admins shouldn't see the application).

These are just a couple of questions worth answering.

In general, will it be a problem if ""Just for me"" is disabled?

Depending on the answers to the other questions (assuming the application will run fine for any user) I'd think it's probably safe, but it also depends which operating system you're installing on too.

Some of the more recent OSes such as Vista frown on certain designs (such as requiring UAC for certain functionality) or not allowing modifications to HKLM in the registry. IN other words, you'd better check the application will run nicely for all users (including non-administrators)."

Sometimes the others users don't use the programs you install. You don't want shortcuts to be placed on the desktops of the other users or the program to be started when the others users logs in.

"This concept seems to make a bit more sense on Mac OS X, where you can keep plug-ins, fonts, applications etc all separate for each user.

So I for example may choose ""just me"" for a bit of software that adds some advanced features, whereas I would not want other users of the machine seeing it."

"Depending on the program you should be able install if using ""just for me"" in each users account without much more space being used. In theory all customizations should then be stored in your individual ""Profile"", either in the registry or some personal area.

Several effects come to mind.

A person who does not have their registry polluted with installation stuff tghey don't need, generally runs faster.
People who are using different applications which may require different system settings
( for example the default ""find.exe"" in WIndows vs cygwins ) don't step on each others toes.
People who require different settings on their applications don't step on each others toes.

Note: ""Different"" people here can actually be the same person wearing different hats.
For example, building vs developing vs testing."