The removal of skills that fall outside of use in an optimized rotation limits the creation of gameplay content to that which can be handled only by an optimized rotation. This has a streamlining effect on the challenges and experiences players have access to, and taken too far, can lead to all classes losing their class identity and feeling like they're all functionally the same. As I understand it, a certain other MMO has been struggling in this regard for a couple expansions after stripping out significant numbers of skills and talents from every class.

A sufficiently complex RPG world should have room in it for spells that have suboptimal damage utility but do something unique from the other spells a class can access. I likely don't speak for everyone on this but personally I really enjoyed the Logos actions in eureka for introducing exactly this - a wide array of utility skills which allowed playing a familiar class in new and interesting ways.

To compare to another IP, the reason games like Morrowind and Oblivion are memorable to such a huge audience has a lot to do with their inclusion of a magic creation system that's fundamentally broken, both in its capability to create entirely overpowered spells, but also in its ability to create things that are hilariously bad. Players remember the scrolls that let you jump super high but don't last long enough to survive the landing, for instance. Nobody remembers creating a fireball that does a thousand damage.