Although both commands appear to be related, you have to see them in their respective scope:
/echo creates a linear output that may be logged. A new line is supposed to be definite - add it to the record if you like.
/aline however comes with all the other /*line commands, even if it may be used for the same purpose. How shall line insertion, -replacement, -deletion or -recoloring affect your logfile? /*line commands allow you to manipulate the window buffer constantly and everywhere. It's consistent that you can capture the content of that a buffer only as a whole, with /savebuf, but not a log. So, even if you can apply both /echo and aline commands on the same custom window, they are subject to different principles.