Saw an XF wagon with a nrside blow-out which was running front axle super-singles other day on M4. The angle the unit was listing at was astonishing and was amazed to see part of the corresponding front bumper shattered What a violent way to come to a halt. Cost or repair must’ve been a shock too!!
Apart from looking ugly they add untold extra stress on the drive and steering gear and also rolling friction and leading edge drag are also increased massively compared.
Here in the states, super singles are less than 1% of the tire market for very good reasons, duplicity being the biggest one. I’ve had drivers blow a trailer tire at midnight in some place like western Oregon and the driver had no choice but drive it that way for 100+ miles to a truckstop. The only other alternative would be a very expensive callout from 70 miles away that would hold the driver there for 4 hours or more. I once had a driver continue 250 miles on one blown tire on Easter sunday in the early hours-nothing else was available!
Understand what you’re saying Alex.
Do the trucks still carry spare wheels over there? Over here for visiting certain European countries (Neil-Coffeeholic will be able to tell us which) you have to carry spare wheels, but in the UK most of the guys now rely on the cellphone and breakdown service. If you run 385 super singles on the steering axle and the trailer, and 295/315 regulars on the drive axle you only need carry two wheels.
Now the big new thing is super single tyres for the drive axle, that’s another size to carry - much easier to pick up the phone if you’re running in western Europe 
Craig
No, a tire (tyre?) adds weight, and most drivers wouldn’t know how to change one, anyways. Today, even most owner-operators don’t carry a spare, unless they have something unusual, like perhaps Dayton rims.
I don’t carry a spare and haven’t had a problem in any countries in Europe I think as long as you can prove you have tyre back up the authorities don’t worry if you have a spare or not. I know I was stopped in Spain and they questioned my lack of spare, I showed them my Michelin Euro Service Tyre card and that was acceptable to them. I have no intention of changing a wheel at the side of a busy road so no point in carrying one. 
The one thing I dont like about them is the fact that the firm where we park had 2 motors go over from SS front blow outs. Ive drove them before and been OK, in fact, the EuroStar im getting is fitted with them, but id sooner have the normal tyres.