Instead of creating a straight vertical wall at the border of a quarry, the quarry should create some kind of reversed pyramid patterns / stairs like in this image (CC):

The wall heights (height between each stair level) should be configurable (in cfg file or when placing the quarry). The default could probably be 8. So with a rather small quarry one could reach bedrock.

Such quarries would simply look nicer and one wouldn't die immediately when falling into such a hole.

This seems more realistic and convenient for players, but it reduces the profitability of quarry.

Let me suggest to discuss this on http://www.mod-buildcraft.com/forums/forum/feature-ideas/ and see how much traction this gets. I'll be closing this issue from github from now, as it should really be for scheduled work as much as possible (there's still a few non scheduled ones around that needs to be cleaned up ;-)