One of the reasons behind my recommendation to install the Russian script on your system is to turn on the Keyboard menu. Since you will have to switch between different keyboard layouts in the process of working with the Russian language, it is very helpful to have the Keyboard menu available, as opposed to using the Keyboard control panel every time you need to make a keyboard layout switch.
Keep in mind that by simply enabling the Keyboard menu without installing the Script, you will not be able to see any layouts in the Cyrillic range (see below).
However, this is not the only function of the script. It also provides ability to change the fonts and keyboard layouts simultaneously - once you change, for example, a font going from Roman script range into Russian, the keyboard layout will be automatically switched for you. There's probably more to the Script, but this discussion is beyond the scope of this page. It is my personal recommendation to use the script. But if you're feeling brave...
Submitted by Jason Mueller:
The MacOS comes with the ability to list keyboards available in the system with a pull down menu without a script. This is a feature that is turned on in all international Macs, but is turned off in all US Macs.
"You state that all non-US Macs have the keyboard menu active. That's simply not true. Maybe all non-Latin have?" - B. Philip Jonsson