inessential 

Used of a work of art, a production that adds nothing of interest to the creator's portfolio. Does not necessarily imply that the work is bad per se, though mediocrity at best is often assumed. Often only of interest to fanboys and completists.

U2's most inessential work has got to be Pop.

inessential by Lady Csyde December 3, 2005

nessential 

Not only necessary, but also essential.

Air is nessential to life.

nessential by MAGICXX1 December 24, 2009