Adding a roster spot solely as a negotiation chip might degrade the quality of play slighthly, but adding a player in order to allow more strategic substitution without worrying about running out of players, might improve the quality of play.
Teams carry substantially more pitchers than they used to carry and it has not necessarily degraded quality of play. One could argue that top starters sometimes get pulled to soon, but in general fresh relievers pitch better than tired starters. One could also argue that bigger pitching staffs have contributed to higher strikeout totals and has necessitated all or nothing batting approaches. That might be considered a degration of quality aesthetically, but not in performance.