This seems to be a fallacy in the mac community, I would agree that to some extent macs do sometimes use better components, and are often better designed, but as you say no one has proved that the different manufacturers use the same (or different) parts, and you also say "it's a very good chance that Dell is using cheap parts in their computer", but again there's no way of proving that.
The biggest parts of these computers will use the same off the shelf components, HD, Optical drives, CPU and GPU and RAM (sometimes varying in quality). There may be differences in chipsets, but as I stated earlier Apple has been just as guilty of using lower grade components in the past to cut costs, or outright faulty components (think Nvidia GPUs).
Before the intel switch it was a lot easier to argue the case that Apple used better components, because there was no real comparisons that could be made, but those comparisons are easy to spot now, and all too often just making blanket statements like other manufacturers use lesser components is untrue.
-----------------------
No matter what happens with the mac, as long as the iPhone farts people will be happy
