Hiyya drwindows. I can see your point, but in my opinion, it's not faffing about. I'm doing something that I enjoy doing (messing about making stuff and looking for ways of improving it) and I see it as being environmentally sound.
It's a learning experience, and I just LOVE learning new stuff.
The economic savings aren't great, but are significant. For someone starting out in a small way, they would save the entire cost of an RO unit. I certainly wish I'd known about this before I started. I would have gone down this route straight away and invested more heavily in storage - say three 1000 litre IBCs.
So far, I've only cleaned my own windows with the raw rainwater, so I don't know how it will perform on other people's windows. I'm going to experiment tomorrow on friends and neighbours' windows and see what the results are. If they are good, there are no ongoing costs! If they aren't good enough, I may have to run the water through a DI unit which will result in small costs. (as the tds is pretty low.)
One thing I will certainly need is more storage to allow for rain-free weeks. I guess that if I have about 10 days supply, I should be OK. I can always use my RO unit if I do run out.