I'm Korean, we don't use 아흐레, 여드레, 이레 usually.
we only use 하루, 이틀, 사흘, 나흘, 닷새, and 열흘.
Would it not be more easy and natural to use a form with Sino korean numbers + 일? Seems way more practical for non-native speakers to me.
A language does not suit itself to the needs of non-native speakers ;)
That being said, as some native speakers have mentioned in this thread, these forms are not used very often anymore (except for the one day 하루 and two days 이틀).
Actually, that happens a lot. It's why English lost noun gender, greatly simplified the plural ending, and lost grammatical case. It's also the reason for creoles and pidgins. Languages love to simplify over time when exposed to significant amounts of foriegn speakers, however mass media has really interrupted the natural processes of languages including the rate of change. We'll see what happens to spoken Korean in the 21st century.