Well PLD's DD potential with full buffs isn't abysmal using ambu lance or Caladbolg if you have DM augs, or just really good augs. It's just more a case of anything PLD can do in that regard WAR and DRK will do better. So I guess your mileage will vary. You can probably complete a large swath of content even if it's suboptimal, but of course a pure DD will always outshine it. Also your Subjob options are fairly limited.
Most advocate for doing something else because it's like trying to look left and right at the same time. PLD, unlike RUN, just isn't designed for non sword/board tanking while doing Hybrid DPS... its' more of an all or nothing thing on PLD.