I want slave sites to have separate image folders (instead of symlink) but template settings do not allow that. Why? Is it really necessary?

YES JMS can do that.

Which JMS version are you using.
This is introduced in JMS 1.2.6 or higher.

See the JMS Templates and the "files and folders" tabs.

1.2.12

well, i can actually edit multisites/config_templates.php and remove 'readOnly' => '1' from
'images' => array( 'action' => 'special' ) but that looks more like a hack

To copy "templates directory", you have to defined a JMS templates in which you specify a templates path and also go in the "files and folders" tabs to select the action "copy" beside the templates directory.

The line that present here is the "images" directory that is only applicable when you replicate a DB as this is explained in the tutorial video CP-05.
If you don't replicate a DB but want to create a fresh slave site, you can not have access to this parameter because the DB is not yet created and the parameters must be also saved in the DB.
This is clearly explained in the video - listen carefully all my commnets in the video.
I have also re-explain that several times in the forum.

Sounds more complicated than it is.
With default setting it creates symlink for images folder - and i don't like it
With hack - it makes copy of images folder - and i like it. That's it.

Btw, no problems with template folder.