True in the case of Rider Swap, but here is my perspective (joining the trip down the rabbit hole).
Couple with a child to small to ride and no FPs:
Parents A & B want to ride SM, but their kid is too small to ride. In a park without RS, the kid would have to stand in line with the parents and then stagger the parent rides. Instead, with RS, Parent A gets in the SB line while Parent B (who gets a RS pass) goes and does something else with the kid. Parent A basically stands in line for the normal amount of time - let’s say 40 minutes (clearly not a liner). Parent A is waiting on behalf of both of them. Then, Parent B goes back to ride, using the FP line. So the wait time for Parents A and B is effectively the same (Parent B just waited with the kid instead of in line) with just a small amount of time taken to actually do the swap and walk through the FPP line added on.
Couple with a small child with a FPP for both parents
Parent A rides SM using a FPP and gets a RS for Parent B who also has a FPP. I think the RS is still useful because then you don’t have to worry about making sure that both parents make it on in within the FPP window. In this scenario, Parent A waits 10 minutes or so while Parent B entertains the kid. Parent A is waiting the same 10 minutes that Parent B would have waited if the kid was not there. Then, after riding, Parent B goes through the FPP line and rides.
Couple with a small child with a FPP for only one of them:
This is where things get fishy. In this scenario Parent A has a FPP. Parent B does not. It Disney does not require both parents to have a FPP, they are essentially giving away a free FPP. Parent A will use their FPP and wait the much smaller 10 minute time period. If riding together, Parent B would not have been able to use to the FPP line. Parent B will entertain the child and then also get to take advantage of the very short wait time that Parent A’s FPP got them. In this scenario, Parent B did not have to wait very long for Parent A to return, nor did Parent B need to wait for the ride itself - despite not having a FPP. So what should have been a 40 minute wait for Parent B without the FPP is now negligible.
Now all that being said, the place where I see issues with the system in general is in how they use FPP line. At other parks, I’ve seen RS handled through the exit so that the second rider essentially rides immediately. FPP lines aren’t always really short waits - depends on the SB time.
I can only assume that Disney originally planned and set it up so that everyone would need a FPP if using RS, as that is what they have enforced on headliners. My guess is that CM discretion (since they are aware of current wait times and capacity), particularly on rides where SDFPs are commonly available, is what allows for the frequently used alternative.