Firstly, it allows mixed pressure supplies, which are very common in
the UK with cold mains water and gravity fed hot water.

Secondly, as the main drinking water tap, it prevents contamination
from the hot water (which may have been sitting around in a tank
filled with dead rats and sparrows).

yes, but i thought that these concerns could be dealt with by suitable
modifications under the sink, so allowing a single-flow to be
fitted...?

Well, part 1 can be dealt with by a pressure balancing valve. However, this
will reduce the cold pressure to the poor lowest common denominator, the
gravity hot.

Part 2 can't be dealt with, although it can be mitigated by taking a long
run of cold before using the water. If you've got skanky gravity fed hot, it
is going to contaminate the cold supply in the internal mixing chamber.
Obviously, you probably won't get ill, unless you really do have a dead rat,
or leggionaires disease in the tank

Basically, unless I had mains pressured and fed hot water in the kitchen
(which as it happens I do), I wouldn't attempt to find a internal chambered
mixer tap. My selecting a good design, you'll find external mixers arent so
bad. It is the cheap horrible ones which have two streams which don't
actually mix mid air.

Christian.