How to never have to worry about being unwanted/unnecessary:

Step 1: Level and gear a variety of jobs that catch most/all niches in the game.

Step 2: Don't suck at them.

Step 3: Bask in the glory of having at least 1-2 jobs that everyone wants at any given time, and thus the ability to get any kind of NM/mission/item you need with minimal effort.

Step 2 is incredibly important, though. I'd rather not have a WHM at all than have a WHM who sucks. At least in the former situation you know you won't be getting cures; in the latter it just comes as a surprise.