Apart from the possibility of tank water getting back into the mains, the
lack of this separation in the spout opens up the possibility of the tank
water being drunk inadvertantly:
run the hot water for washing up
turn tap off
come back later for a drink, the first water to come out of the spout when
you turn the cold tap is formerly warm water from the tank. If you run the
tap for a second first, no problem, but how many think about this.

I always let the water run a bit before I drink it, and the said water is
from an under sink filter in a single tap!

I would be surprised if most people place glass under tap, turn on tap, wait
for glass to fill, then turn off tap
Most people I know will turn on tap, move glass under it, move glass away
then turn off the tap.

Sparks...