I totally get why a van mounted system works for many WFPers, and in no way am I saying you shouldn't go for it.
You will, as has been said use a lot more water to start with, rinsing away all that accumulated soap from in the seals and dirt from around the frames, etc. So at first the trolley can be a pain having to refill it or put another barrel on more frequently. But once done, you will need a lot less water, so trips back and forth to your car/van become much less frequent. If you have to keep getting it in and out of a van to do detached houses that are spread out, I can understand how that might be a pain, especially if your trolley is heavy. But if it's a light one it takes seconds and is easy. I modded mine as I described earlier and it can be lifted out with one hand now with ease.
My work - and I can't be the only windy who's work is like it - is not suited to a van and reel system. Parking in many of the places I would need to park would cause a major obstruction for other people. Much of my work is also well away from road access. Using a long hose attached to a van would be very problematic. I would be dragging the hose across other people's gardens and for much of the time I would not be able to see much of the hose and whether it was creating a hazard for pedestrians. Using my trolley with 15 or 25mtrs of pole hose gets me to all of my work and is a doddle. 25ltrs of water lasts me about 45 mins. It never takes more than 5mins to grab another barrel and carry on.
You would think that if I can work with a trolley then I could work with a backpack on a sack truck. I could, but for reasons I'm not going to go into here it would be a lot more difficult than with my setup.