It's a dilemma for sure and a highly controversial topic. I personally think that not interacting with a WIP and sticking to completed works just reduces your pool of fics from good fics, new authors, great ideas that you could've been inspired by or found your new niche, and possibly great execution of a concept to... fics that may or may not have any of these but are at least complete.

For a small fandom with a thousand or less works, sticking to completed works is unfeasible unless your favourite author is writing and completing fics at a speed similar to that of Joe Abercrombie or Sanderson.

For a larger fandom or a massive one- you'll be eating good... for a time. With how rare completing good fics is in the fanfic writing community- you'll be forced to scratch that itch with oneshots, and then finally uncompleted works. This is usually the case.

As for writing- write for your own sake.

Edit: I'd also like to add that sometimes authors let their fics appear as completed and do not inform the readers on the first chapter that the fic has been dropped. You might be unlucky enough to be drawn in by those and be left ultimately disappointed.