NHFL9 is correct that SMS can notate your PNRs to 'link' them (e.g., pax is travelling on with a Medallion member) and get your companion(s) seated next to you in Medallion seats.

The one potential advantage I can see to being on the same PNR is during IRROPS. I know when my wife and I were traveling on separate (but 'linked') PNRs a few months ago and had a flight cancellation, we were both rebooked on the same flight, but I was given a seat assignment and she was not. I suspect if we'd been on the same PNR that we both would have had seat assignments.

One other piece of information to keep in mind is that PNRs can be separated but not combined. So if you put everyone on one PNR, it can always be split later. If you start with separate PNRs, they can be linked by annotation, but never truly combined.