I am pretty certain the debate hasn't been about custom powers (I have made a fair few broken power I am not defending them) but powers the Dev's built themselves which should be balanced.
First of all, "cool thing" isn't exclusively about custom powers. It can be anything which isn't plainly laid out in the rules. A character concept, for example, like playing a Fae. They aren't in the player-recommended Templates section, even though Fae are a huge part of the DresdenVerse. They certainly weren't left out for space considerations. They were left out because, all things being equal, they are considered by the canon to be tantamount to forces of nature, and unplayable by most PCs in a way that remains true to the setting.
But that doesn't mean nobody can try. That doesn't mean nobody *should* try. But that's a table decision.
But certainly, those elements have been catalyzing factors. Yet I feel that the drift of threads like this has ultimately been about tolerance and uncertainty (and, conversely, intolerance and stubbornness).
There are elements of the RPG which are left vague. This causes uncertainty. "What should I do?" Sometimes setting (or extrapolating from the setting) can inform the vagueness, but other times it just can't. There are elements of the RPG which are roughly outlined, but as they are ultimately still governed by "Your Table, Your Rules", they persist as guidelines, not rules. The setting often supports these guidelines, but because the DresdenVerse still has so much unexplored territory - and can, by extrapolation, support a vast multitude of ideas - that it can be easy to discount what the setting *has* established, and extrapolate *limitations* from there.
There's nothing wrong with approaching this uncertainty with Hamlet's notion that "there's more things in heaven and earth, Horatio, than are dreamt of in your philosophy". GMs should always say "yes" if there isn't a compelling reason to say "no".
But when there is feedback from community members that something *shouldn't* be, for [insert narrative/setting reason here], don't just dismiss it as negativism. Take the feedback, on its own merits, and if you still want to do go forward with your character concept or rules interpretation or whatever, you're free to say "I see where you're coming from. I can see how the setting/rules can be interpreted that way. Nevertheless, I still feel confident that I can make [insert cool thing] work in *my* game, but I'll keep your feedback in mind. If you have any thoughts on how to do it right, I'd love to hear them."
But you have to understand that, as far as Evil Hat is concerned, THIS forum *is* the errata. It is precedent. Which is why the people in the unfortunate position of saying "no" to [insert cool idea here] are holding the line as strongly as they are: because they feel they will have to live with it if they say "alright, sure, go for it."