I use FontLab Studio, so that's what I have in mind. But other font editors might find some of this useful.
About 90% of the time, when I am editing a glyph, I wish I could edit the outlines and keep the sidebearings in place... which of course means the sidebearing line needs to "float" with and glyph changes I make that affect the advance width. Of course, occasionally one does not want this, so it would be ideal to have some sort of "force linked sidebearings" toggle button when in the glyph window. Maybe the button would be part of the FLS glyph properties floating palette?
Even when the sidebearings are linked to the outlines, you would still be able to click and drag on the sidebearings as you can today, to adjust them.