There are flights which sell out completely, although this is far less frequent since Southwest stopped overbooking flights.
Just for peace of mind, I always book a companion reservation at the same time I make my paid reservation.
The only time this becomes a burden is if you need to change the flight. Then, the companion reservation must be cancelled first (with the tax refunded) before the paid fare
can be changed. Finally, the companion can be re-booked, yielding a new passenger record number for the companion.