While I agree it's good to have variety in the jobs, I really don't feel they're too similar. I actively play both WHM and SCH, and BLM and SMN. They're very different. I also play BRD, it's very different as well.

Like people have already stated, too many differences leads to people being left out. Remember back when everyone was farming Brayflox HM speed runs for Myth? The ONLY DPS jobs people wanted were BLMs and BRDs. Just last night, I saw a Party Finder for Levi EX that specifically stated "no monks." Exclusion is not a good thing in a game like this.