Maighstir: whereas most users likely expect their games to fill the screen rather than pillarboxing them.
Just curious, aren't most GPU options set to "Full Screen" by default (it's like this in NVIDIA control panel and all laptops with integrated Intel graphics that I've come across)?
If so, setting aspect ratio by default to true in DOSBox would change it to the correct 4:3 format, but it would still be stretched to full screen by the GPU for Joe Sixpack. Everybody's happy. "most users" get their full screen, and those who prefer a true image with black bars have their GPU options set to "aspect ratio" anyway.
EDITED for clarification.
EDIT:
Also, most non-DOS games from 1995-2005 come in 4:3 format. If "most users" didn't have GPU options set to Full Screen anyway, they'd be seeing black bars in those games too. Since those games come in 4:3 and we don't hear many complains about them, why not set the older DOS games in this format too?