Love the FCFS feature at NM state parks, now if they would just allow overnight camping, alas our governor is not an RVer. We have to go to TX or AZ to enjoy state parks, which we have done over a dozen times since April 2020, hopefully end of this month they will reopen.
Having all sites reservation only allows for the trickery mentioned above, for those that have families and work fulltime FCFS gives them a chance to enjoy their RVs on any given weekend including major holidays, where an extra day off is the only way they have the time.
Another nice feature is that we can reserve same day of arrival, when we get to NM state park we drive around and choose a site we like, then use our smart phone and lock it in. Sure we use google to see RV sites beforehand, but being there in person lends the best view by far.