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-...