Yeah but that's the problem... I don't want a special mystery Dell ISO, I want to use stock Ubuntu and have it Just Work. Using Dell's ISO locks you in to that certain image (upgrades are very likely dependent on Dell), it locks you in to Dell support (good luck posting in the Ubuntu forums about your super-special Dell ISO), and it almost certainly means they're not contributing back to the OSS community (though that's not so much a pain point as it is a general gripe).
Why do you assume it's so "super-special"? I doubt they'd have the rights to call it Ubuntu if it didn't have the standard package manager configuration pulling updates from Canonical.
The OP's argument is precisely about the fact that they shouldn't provide any ISO at all!
Just make sure your ultrabook is Ubuntu-certified or something and let people install the official version.
They could just get the hardware Ubuntu certified, literally. It might actually cost Dell less, too. Of course everyone wants their own "ecosystem", so maybe they'd rather just lose the customers if they can't lock them in.
They look nice, maybe you should get one.