Hello Michael,

So here's a 2013 version. I don't think there is indeed a specific attribute name like "surface_id" supposed to be used for that purpose.
However I just came across this example (maya to unity) where custom attribute is created and stored in .FBX file (even animated in this example) ... so it's not completely odd to store additional information with that format.
https://www.techarthub.com/how-to-import-custom-properties-from-maya-into-unity/

Having vertex merged is kinda useful to actually exploit "true" connectivity, like assigning materials or groups. In many polygon modeler it's common to simply double click on a polygon to select the whole connected mesh ... like a bunch of bolts that doesn't have any group/name set yet, you can just double click on them and tag them.
With the "unweld" options I have tons of individual surfaces (on purpose), very handy for uv'ing, but not so much for 'normal' selection work.