Now that we have the flexibility of segments in Analysis Workspace, pulling unique visitors/visits would make a lot more sense. Back in the olden days of SiteCatalyst 14, this flexibility was not there, and required a very specific type of implementation to set up.
If I wanted to (technically and truthfully) pad my numbers, I would take the raw event number. However, if I wanted to be more realistic about my numbers, I would create a segment where the registration even happened, and report the number of unique visitors it had.