[supervacuo]

An omnivore eating less meat is good for all the reasons being vegetarian is good - it's not all-or-nothing.

That more-vegetarian-than-previously omnivore calling themselves vegetarian is helpful to other vegetarians in that it raises the profile of those issues and normalises pro-social dietary choices.

It's obvious that people making huge lifestyle changes struggle - I'd say support is more personally and societally useful than sneering disparagement about them not "actually" being vegetarian. Technically correct: the worst kind of correct.