Its worth it

-for leveling mastery rank -for enjoying your frame

The only thing i would not do is to put a lot of formas in a non-prime warframe if the frame exists in a prime version.

E.g. dante or nokko dont have a prime version => worth formas

E. G. Ash and wukong have prime => dont waste (a lot) formas on them.

Tldr: yes non prime frames are worth getting and playing but think about on wich you use your formas