I am a big fan of rear fog lights (I am originally from Scotland and we get plenty of Fog there!). However, the problem here in the US is, that people here drive with their front fog lights on a lot of the time as 'driving' lights.
They pull/twist/push (delete as applicable) the foglight switch to turn them on and then leave them like that for the life of the vehicle.
Then they step into their new Audi or BMW with rear foglights, and twist the fog lights to 'on'. Well the Audi and BMW have 2 steps to the foglight 'on', the second step turns on the rear foglights. Well these people who haven't had rear foglights before, set the switch to position 2, and have their rear foglights on all the time.
There is an Audi A6 that drives around here all the time with his rear fogs on and it annoys the [email protected]
This isn't an issue in the UK, as it it illegal to drive around with your front driving/fog lights on if there is no fog. And I should know, I got ticketed for it! 'Excessive use of lighting' was what I was written up for. At the time all I could think of was 'hmmm, getting written up and fined by the police, for being TOO VISIBLE on the road...!!!'. But the law is the law!
However, to answer the OPs question, it would be cool to have them on the US spec XK, as I think they are a valuable safety feature.