Contra: Is that strictly necessary? The menu is already overcrowded.

Pro: Yes it is. Have a look at the page hit ranking at distrowatch.com :
     Manjaro is at number four among all linux distros.

Contra: But isn't it just a temporal phenomenon?

Pro: No, it isn't. Manjaro is at number three as to the 12 month average.
     It has a competent core team and an engaged and constantly growing
     community.
     The core team also works as one of the main contributors at the project
     calamares, the highly customizable and distribution-independent installer.

Contra: But it's an arch derivat and arch already is in the menu.

Pro: It is an arch derivat but unlike arch manjaro releases several different
     precisely preconfigured BINARY and TESTED desktop editions, three 
     core editions and several further community editions.
     They all run reliably out of the box (life and/or after installation).

     The KDE edition is one of the three core editions (and besides Neon
     one of the most popular and stable KDE-based distributions that 
     actually exist.)

     So if a Manjaro user/developer files a bug for a (binary!) KDE package this
     is comparable to other binary distributions and has a higher significance
     than a bug filed for a self-compiled arch package (unknown compiling
     options ...). Therefore it's worth making a distinction between them.

(Additionally one can take into account to add another menu 'Branch' with
contains the entries 'stable', 'testing' and 'unstable' in order to
get the Platform-menu more clearly arranged.)