eah, but wait till you have to bind an xref and it's nested in a drawing, which has a nested xref, which has a nested xref, which has a nested xref etc etc.
I've seen that and it wasn't pretty
When you bind the parent xref, all the others come along for the ride...simple! There are two options for the bind command....bind and insert. Binding keeps all layers from each child xref separated...you have a "part-01" on two xrefs.."A" and "B". The layers would be "A/$part-01$" and "B/$part-01$". With the bind option, both would be on "A/part-01" layer. A little tricky at first, but easy to get the hang of it.
We use attach exclusivly...all our stuff is assemblies and sub-assemblies. When I do a site layout, it includes the gas handling system, flare and the interconnect piping. In the GHS I have a control cabinet, blowers, piping, the skid and instrumentation, all of which are xrefs in the GHS, and all of which must show on the overall site plan. With overlay most all of this would not show.
Basically, it all boils down to how/what you are drafting. An archi would find overlay very handy in creating the various prints for each dicipline...electrical doesn't need plumbing who doesn't need....... For us however, attach is the only way to go.