You make a great argument, and I agree with you that it is very unlikely that a model which can only ever be an approximation of a human brain will ever exhibit intelligence.
My gut tells me that we'll have more success building an exact 1:1 copy of a brain, but from digital components rather than wetware. The discovery of memristors, and other yet to be discovered building blocks, may lead to just these types of advances.
If consciousness still doesn't emerge in such a 1:1 copy then that would be spooky.
Making a 1:1 copy of the brain without understanding its mechanisms is pointless, because there's no way to bracket the problem. You don't know how low-level you have to go: if there's only a single crucial quantum interaction, you have to model that level as well, increasing the complexity many orders of magnitude (if you have to model quantums or atoms, the simulation wouldn't even fit on earth I'm afraid). Similarly for the higher levels: it's obvious that brain development depends crucially on a stimulating environment (and meaningful interaction with it), so are you going to simulate a complete environment and upbringing? Reminds me of the Sagan quote "if you want to make apple pie, you first have to invent the universe."
In a similar vein, we have succeeded in copying (digitizing) the full human genome. But, to actually do something with that requires an even greater tasks of understanding what all those genes in it actually mean (what the proteins they express do). For the brain we haven't done the former, let alone the latter. I believe there's more point in trying to understand the working of the brain or mind on a more abstract level.
I agree with you that an more abstract understanding would be more useful than a naive copy, however I think calling it "pointless" is a bit of an exaggeration. Imagine being able to make an indistinguishable 1:1 copy of your own brain at the exact moment at which you die, voilà immortality. Tad Williams wrote an interesting bit of fiction about exactly this called the Otherland series - well worth a read.
Not to mention that simply building such a copy would yield its own set of insights.