I have 5 capped toons. All of them excel at their role. None are TRd except my Artificer experiment.
If you spend the time you'd spend doing completionist getting the perfect raid and epic gear setup instead, your character will be better anyway hour for hour for the time you spend.
I might do a completionist, after I get every class I want to play to cap, finish my max damage artificer TR, etc etc etc. There are simply things that give you more bang for the buck that take less time than completionist, at least from my perspective.
It's not a complete waste of time to do completionist, there are just easier ways to make a character powerful in less time.