Plesk is nice, it has a nice interface. However, it leaves much to be desired on the Linux end. The same features have been forced down our throats for the last few years and Parallels refuses to introduce any new feature due to "architectural changes..."
We all know that software development will require some architectural changes from time to time. Unfortunately they don't want to commit.
Plesk does allow you to use some remote services (e.g. database), so that does make it more attractive to the system admin that wants to setup the traditional environment, which is to keep things on their own dedicated servers, not cram it all into one machine... after all, we're no longer in a shortage of server hardware, etc.