Carl:
alot of dutch and belgian firms that do uk regular have them because of our axle weight restriction a 4x2 dutch outfit will be legal over there but as soon as he,s off the ferry over here its over weight so much for harmonisation on weights 
i quite like the little wheel 6x2s sb components do sideskirts that just cover the wheel
They had a brand new Lomas Distribution Actros on their stand at the Commercial vehicle show with the side skirts. It looked really smart, probably the best example of covering the mini wheel up.
N14ERF:
Wheel Nut:
papermonkey:
Yep, all of that and cheaper tyres tooā¦ching!! 
Not as much difference in price as you may imagine and a rarity value added to the price.
You cannot quote exact tyre prices as everyone pays a different price, but a quick Michelin search found 17.5 at Ā£375 and I am fairly sure I could buy a 22.5 for Ā£400
I buy 17.5 contis for aboutĀ£180 and have just ordered 4 firestone( bridgestone) 295r22.5 for Ā£289.
I buy ftp dafās to pull curtainsiders for at least a 29.5 tonne payload, paid by the tonne so i want max on the back.
I did mention the disparity between tyre prices, my point was to show that it is not simply done for price, but for the reasons you state, PAYLOAD.
Which carries me on into another thread, if hauliers want fuel economy, profit and payload, why do they put trailer tyres on the steer axle
donāt forget Klunk has stabilisers on his FH as well and he pulls a heavy fridge 
Mr B:
If they are cheaper and lighter and allow bigger tanks or higher payloads surely the question should be why do most 44t trucks have standard tag axles?
Every coin has another side to the face.
Bad points, eats tyres (the smaller size means that it does twice as much revolutions.
Expensive in maintenance, we use a hell lot of airbags, brakes etc.
Non standard, so repairs outside main dealers can be challenging
Lighter build makes it more affected by wear.
Resale value of the truck is less than a full axle.
Many hauliers / Drivers didnāt / donāt believe in them, but after a slow start you see them more and more, we started in 2003 (but they where out there already much earlier) to use them and and everybody was laughing at the look, but now do you really not notice them any more.
Even traditional twin steer companies are now using them.