I agree. My wife and I are both Plats but we still separate our PNRs. My reason for doing so is based on the following hypothetical situation:

Two people (with the same Plat status) are booked on the same PNR. At the Platinum window there are 2 seats in left in FC, but only 1 V seat. Assuming the two Platinums are the ONLY platinums booked on the flight, I think they'd still be passed up for the upgrade since there's only 1 V seat available. And that 1 seat would go to a Gold at their window, leaving only 1 seat in FC. At the gate, that leaves both Platinums trying for that one last seat = no chance of both Plats flying together in FC.

On the other hand, the two people (with the same Plat status) are booked on separate PNRs. At the Platinum window there are 2 seats in left in FC, but only 1 V seat. Assuming the two Platinums are the ONLY platinums booked on the flight, 1 gets the upgrade and the other does not, leaving only 1 seat in FC. At the gate, that leaves the other Plat trying for that 1 FC seat = much better chance of both Plats flying together in FC.