Yeah, you don’t NEED infinity, but it allows you to actually play the game properly and efficiently. Why do I only run the pit and eldritch/shenk/pindle? Because those are the areas with the fewest immunes. My character(trapper) can actually solo chaos and baal in a one player game by casting cloak of shadows and stunning the immunes while my merc kills a few and then death sentry cleans up the room, but that isn’t really a fun playstyle and there are some immunes like souls that are immune to both light and physical.
Having an infinity would allow me to actually have fun and easily clear most lvl 85 areas in the game in players 8. Right now it’s only really efficient farming the pit in high player games. One player games are hardly worth it as almost nothing drops.