Greg writes:

I know it is his mortar and his wall, but I would apprecriate some
corraboration that his miserly habits may result in unstuck stone
falling on his beautiful Italian tile floors.

The only reason not to reuse spilled material is if it is old. Once it
starts to set, it won't be as strong in bond or compressive strength.

Use a polymer admix, like an acrylic emulsion. Well worth the extra cost
in bond strength for wall applications.