Hi Edmund,
I understood that this was a question in regards of clarity,...
When it comes to symmetry, that is another factor witch can possibly be affected by an extra facet depending on its size and location.
As I said I would need to see it.
Extra facets are not unusual to see and you are right, the cutters when maximizing their rough sometimes go back to remove a natural if it improves the diamond's value. I don't think this is unfair, however if you feel you are being cheated, look for a stone without the extra facet.
Please do not confuse this with some Russian cutting styles where extra facets are the key to their unique cutting diagrams .
If the diamond you are looking at is GIA certified and has an extra facet listed on the grading report with symmetry graded as excellent, it most likely is. There are other factors that are more important to the symmetry.
If the extra facet wasn't listed on the report, it is an indication of it being added after the stone was submited to the lab. This would however make the cert no longer valid.
George