It would be like selling a Jaguar with a Ferrari engine; still a weirdo, but at least fully functional. You can't seriously claim Mobile Webkit to be to a (theoretical) Gecko-on-iOS like a Fiat to a Jaguar.
And people are not "too dumb to realize". The whole point of the HTML standard is to make sure that all browser render the same. The whole point of the standard, and all the efforts of all the browsers, go towards a single goal: making all pages render equal on all browsers, that is making people NOT realize there is a difference; the technical problem of different codebases exhibiting different behaviors in rendering is just that, a technical problem to be solved.
I maintain that the developer-centric position of focusing on Gecko and thus refusing a iOS port as "useless", is actually doing Firefox lots of harm in its adoption. Firefox is the product, not Gecko.
Moreover, even if Apple eventually lifted the restriction, you want to be ready that day; you would still have to port the whole browser in addition to Gecko, including non-trivial issues like the extensions, and coordinating a solution for using extensions on all mobile platforms require years; the ecosystem of extensions need to adapt and evolve over the time. Releasing, maintaining, optimizing and evolving a Webkit-based Firefox on iOS would be still a gigantic effort and would still bring benefit; you're then free not to use it, if you only need Firefox as a way to use Gecko.