I just installed rar on a new machine, then tried opening a .rar archive with ark and got the "no suitable plugin found" error message.

Installing unrar as well, the archive can be opened. 

Ark shouldn't require the user to install unrar if rar is installed, it makes little sense to have both rw and ro versions of the executable in the system. From what I see, even the syntax is the same, with unrar missing the commands related to archive creation.