I recently upgraded my memory, but I notice that while the OS does NOT page out at all (from what I see in Activity Monitor), the machine pages in regardless.
Why does that happen? Is that a problem?
This is normal, and you couldn't do anything without it.
Paging in means loading something from the disk for the program. It does not have to be something that was previously paged out. For example, when you open a program its code has to be loaded. If you haven't used it recently, it's probably not in memory, so it has to be loaded. This counts as a page in. In order to not have any page ins, you would have to prevent the system from running any process, including those which are part of the boot sequence.