It’s an issue when you’re not in the same place for more than a couple of nights, as the clothes need to dry properly.
I’ve done laundry on a holiday before, but it’s a bit of a pain to find somewhere in your itinerary if you’re on a road trip or interrailing, for example.
(I don’t think I’ve been on a holiday where I’ve stayed in the same place for more than three nights in years though)