Well, leveling a combat class is necessary to complete the vast majority of quests in the game -- including main story quests, which unlock core features like retainers, grand companies, your chocobo, etc. FATEs are purely combat-oriented. Every map outside of a town is littered with enemies that you simply cannot deal with directly as a gatherer/crafter. So in that respect, there is simply no contest -- DoW/DoM is needed for most content in this game (and is rewarded far more), whereas you can't argue the same thing for DoL/DoH.

If you're talking strictly about mastering only, I don't know. I'm not at max level yet. Does anything important unlock at 50 besides more dungeons, raids, and PvP modes? Seems like it is just more combat, so if that is not your thing and you'd rather craft, then putting energy into mastering a combat class first is a waste of your time.

This is a video game; you should enjoy it as you like, and not according to what some scrub "friend" considers "pro." And yes, him calling anyone who thinks otherwise "noobs" is insulting. Honestly, he sounds very immature and annoying to talk to. I don't see why it's worth even engaging in a "debate" with this person.