Some of us just don't want to pretend and play like there's a class system all over again. I want to truly feel that party synergy in combat. Like it really does matter having a cleric, wizard or rogue around and it's not just another cosmetic thing like all the rest.
This is what makes a good class system. I want to recruit a companion and bring him along to explore the underdark and really feel the difference in my playstyle and be rewarded for adjusting to my new toolkit, and hopefully, be encouraged to steal his class identity for another playthrough or be excited to plug in a few different companions to improve my party synergy further.
