Dimension in Ai are live and follow the objects they we snapped to (only if NOT get moved on their own or together with the objects, which is stupid and should be fixed: http://illustrator.uservoice.com/forums/601447/suggestions/47739104)

But another problem is unbound dimensions, that don’t have an anchored parent anymore.
Ai offers nothing if I delete an object that has a dimension attached to it
It won’t offer me restore the link if I paste it back.
If a dimension was bound to two objects, and delete one, there is no way I can move the second one and keep it live, it gets expanded immediately... There is no way for me to reattach the loose end of a one-legged dimension to another object.

At least offer us to delete these automatically, and offer an ability to not show the choice again and reset the warning, if needed.