Every UI scaling mechanism I have seen in practice has some weird artefacts. There is a reason UI designers still create their icons as bitmaps and not as scalar graphics. How do you scale a one pixel hairline by 20%? You either use aliasing which changes it from sharp to a bit blurry or you leave it at one pixel which changes its proportions relative to the rest of the UI. UI scaling will start working (from a visual point of view) when the thinnest UI element is multiple pixels large.
Back in the real world, it works fine. Sure you can make up dozens of scenarios where it doesn't, but as we all know, theory rarely translates into practice.
Again, problems that were faced decades ago and solved decades ago.
What financial incentive would Apple have for continuing to use the A4? I would think it would be cheaper for them to streamline their processor manufacturing to a single chip. Both are made on the same 45 nm tech and the A5 has been in production for over half a year, so why not use it in a cheaper model as well?
The A4 probably has higher yields than the A5, thus lower cost to produce and hence, Samsung charges Apple less per A4 than they do per A5. The 3GS did not get an A4 in 2010. Think about that.