James
As Simon Gerrard and a few others have said, truck mounts are in themselves, good marketing tools. I have a rental property to do where the carpets are trashed, and i would think that my portable would not be up to the task. The extra heat, pressure and phenominal suction of the truck mount will trounce a portable on this type of work.
Another fact that i can't get my head around is the high prices that some cc,s charge. With a truck mount, i don't feel embarassed with charging a high premium as as you said, i have spent a lot of money on my equipment. I still use my portable when i have to, but it is a hassle.
As far as life expectancy is concerned, if serviced properly, there is no reason that they could not last ten or more years, with the odd outlay here and there to replace any part that goes wrong. In three years, the only thing to have gone wrong on mine is the fuel pump (£60 to sway, a car one) and that is it.
With all the second hand tm,s coming onto the market, they are affordable. I have seen vans on the road fitted with tm,s that are around fifteen years of age and still going strong.