Glass carriers are a specialized piece of equipment intended to solve specific problems. Sheet film negatives too large to not sag in the middle. Roll film negatives with a pronounced curl - especially the piping variety. Recently I have been printing from series of 35mm negatives that were stored uncut and tightly wound inside their metal film cannisters for 50+ years. That would be the definition of a specific problem requiring glass for the best results.
I have and use a full set of both glassless and AN glass carriers for my D5XL. If one can afford them, they should be a standard tool in every darkroom.