I'm not sure about the dimension layer, but geometry I can understand. In drawing mode, I will use as many skeleton sketches and datums as I can for dimensioning as they are less prone to causing dimension failures in the future should geometry change.
This is particularly important when creating ordinate dimensions. If either of your first 2 attachment points in an ordinate system is compromised, your entire system fails and you cannot simply edit the attachment of the 0 point, one it req'd to delete and start over. Also, it denotes your design intent much more readily, avoiding the issue of selecting an inappropriate line by mistake which is 0.03125" off from your target. Worse yet, inappropriate geometry which moves or disappears on you.
Dimensioning to non-goemetry is much easier if all geometry is hidden. I actually have a default layer which includes all parts and assemblies other than skeleton models for this reason. (I should note here that I do not have access to AAX, so we use a plethora of work arounds for top down design).
