The dimensions are too small for transparency to not look like garbage most of the time, otherwise you have to shrink a character to fit within the bounds of the size limit and make sure they aren't cut off at any weird points, which doesn't matter nearly as much with a background.I never had an issue with them, but to each their own.
It's just not something I care about and don't think even improves avatars most of the time, if the dimensions were like 150px wide x 200px high or something, transparency would look way nicer.