When we create a new dimension with Smart Guides enabled, it snaps to an existing dimension quite fine: by the line, by the anchor, just by proximity. The precision is strong, the snapping has a distinct ledge.

If I try to reposition on re-snap an existing dimension to another one though...

It doesn't work.
See the GIF attached, please.
Illustrator highlights the neighboring line, it tries to persuade you it snaps... but there is no ledge felt, no snapping happens, the line does no stop exactly at the position needed. It’s just broken.

And since we can’t define a default dimension line offset (or edit one that is created for the Alt-mode), and there is no way to make a stepped offset (requested here: http://illustrator.uservoice.com/forums/333657/suggestions/47558036), this prevents us from making sure these are placed with a constant offset — the only soultion is to delete a dimension and create anew :( Boooh.