Oh, I'm unlikely to start disallowing the storebought blockers I already allow, in fairness. And there are casts that fail against competent status practically no matter what (HI, SHINING FORCES. HI, MODERN PERSONA), so. I just don't like extending an exception so far it turns into a complete rule and now shit like Yellow Scarves, which were only good to make Juan awesome in S3, or Cure Alls, which were good for absolutely nothing in AtL2 (how often are you getting statused there anyway?), suddenly become godly in a duel setting.
Not to mention that status blocking, even at its most useful, tends to be somewhat niche in-game, and the transition suddenly makes them the big choice for PC casts in a DL setting. It bugs me for some unfathomable reason, but I just shrug and move on with single statusblockers. Full statusblockers just get more annoying to me, especially given how I hate dummying out equipment (for an example, if FFT's ID blocker with Auto-Life actually let you Auto-Life in a duel, I'd ban it entirely instead of only allowing the ID, and I don't allow dummied Reflect Mails either, for all the good that would do them).