It's popular for some reason even in countries where it doesn't save keystrokes. For example in Croatia we have 5 characters with diacritics (š, đ, č, ć, ž), all of them have dedicated keys on the keyboard and yet many people have a habit of simply not using them.
In German, ä would never be replaced by a (if all you have is ASCII the proper replacement is ae), except by foreigners who assume that diacritics don't matter.
It's really no different from people in English shortening "you" to "u" in texting as well, or "lol". Everybody saves keystrokes wherever they can.