Well, you're using meta-textual arguement instead of an in-universe one, and your meta-text is in my opinion wrong in any case. Spoiler and Huntress play VERY different roles in the Bat-verse, and for that matter Spoiler never really had her reputation for incompetence until DC was setting the stage for the massive character derailment that was War Games.
Spoiler's role vis-a-vis a Robin was largely related to them being romantically entwined, or at least having a fair amount of UST. Giving superheros a superhero girlfriend is fairly standard fare, although Spoiler managed to grow into much more than merely being Robin's girlfriend.
Huntress and Batman, on the other hand, have pretty much never had any sort of sexual tension (which figures considering who the original Huntress was). Even her thing with Nightwing was a basically a meaningless one-night-stand. Huntress has been defined by her ideological conflict with Batman, particularly the Bat-code against killing. She's certainly not a slightly-less-competent sidekick to Batman like you seem to imply; she's always been Batman's dark mirror. It's no coincidence that her current origin has her family being shot to death right in front of her, exactly like Bruce Wayne. If one considers Huntress as a dark reflection of Batman, it certainly seems reasonable to posit that they would be at the same overall skill level, with each having their advantages and disadvantages in particular areas.