I'd argue that it's less pretentious and more helpful to non-native speakers, as it indicates pronunciation. It's a marker that the word is pronounced "co-op-er-ate" and not "coop-er-ate". The diaeresis works much like vowel markers in Hebrew to make the words easier to pronounce for people reading a word they might not have heard pronounced.

https://www.newyorker.com/culture/culture-desk/the-curse-of-...

So why don't they just leave the dashes in, you know, like co-operate? I think a dash is much easier understood than an umlaut. Especially for native speakers

AFAIK it's only The New Yorker that does that. They have their own standards I guess.