Martin, rollers only help you extract maximum power in certain circumstances, I find that the majority of turbo cars I map tend to be knock limited on ignition, so you can find that just as well on the road as on the rollers and I find that most turbo cars are EGT limited on how lean I am prepared to run them, so again that is just as good on the road as on the rollers.
I do find rollers very useful though, especially on N/A cars, I can do a better job of an N/A car on a dyno dynamics set of rollers in an hour than I could in a week on the road, as they arent knock limited and they arent EGT limited generally, and with a set of rollers like yours allowing you to hold fixed speed sites it makes it an absolute doddle to visit every single site on the whole fuel map in a very short period of time.
The only reason I dont do more cars on the rollers is that it increases the cost if I do so, if I had a set free to use though I would do more on them as they defiantely have some distinct advantages, not least of which is that you can hold a car under load properly without worrying about your life or license.
At the end of the day, the knowledge of the mapper is the key thing, and I would sooner have a car mapped on the road by someone like Stud@MSD who is a very good mapper than by most rolling road tuners who arent!
Its just a tool at the end of the day, and cant do the job for you, but anyone who thinks that rollers arent useful at all doesnt know how to operate them properly IMHO