I think this is an okay idea gone wrong.
Assume that I add JOHN DOE to my RootsMagic tree, someone else adds JOHN DOE to their PAF tree, someone else adds JOHN DOE to their FTM tree, and now there are 3 separate and unique _UID's for the same JOHN DOE.
Then we 3 all upload our GEDCOM to Ancestry and all 3 JOHN DOE records on Ancestry would contain a different _UID.
A 4th person comes along and merges those 3 records into their Ancestry tree and gets a copy of each _UID, none of them helpful to him, Ancestry or anyone else.
I am convinced that _UID's on a tree are nothing but useless, distracting, and wasteful clutter.