If they are linked they can be upgraded together at the airport which is better than nothing, but you lose the opportunity to be upgraded at the regular upgrade window with whatever flyer has the highest status — e.g., when my wife is on my reservation we get upgraded for F at the PM window and for C+ at booking even though she is a GM because my PM status pulls it up.

With linked reservations when you get to the airport they can put you on the C+ / F upgrade lists but you are then competing with everyone for those seats (especially on popular routes where you may clear at the PM window but by the time you get to the airport a few last minute DMs have booked and so they trump you on the list).