Of course it does.
︎ People are less likely to roll an alt for farming
︎ People may be more likely to roll on a pvp server, which are already more popular than pve servers, because they can just press a button and go from a perfect pvp spec to perfect pve spec
︎ Some classes have hybrid pve/pvp specs that are good for doing dungeons and good enough for world pvp. Nobody will play those anymore
︎ Heal specs are less likely to group with others if they can just swap specs whenever, for free. I can solo a lot of stuff on a paladin, if I can get into the proper build and gear. Why do I need your help?
︎ It makes the game even more min-maxified, because people are going to be expected to be in the perfect spec for the given content at all times
︎ It removes what is one of the few substantial gold sinks/ inflation reducers that remains after people have bought all their skills, professions, mount, etc.
This has gameplay ramifications too. Have you tried getting to Scarlet Monastery on a fresh pvp server as an alliance toon, with horde running around everywhere? It also minimizes the unique role that warlocks have as summoners.