Hmm, perhaps "essential" is the wrong word. Demos are not, of course, essential. There's nothing to stop you buying a system without one.
But I would definately recommend them, after all what have you got to lose?
I do think that you should ask advice on this forum before spending thousands of pounds on any piece of equipment.
Window cleaning forums are great, and as a source of information they are 1st class. However, with the greatest respect Tosh, - asking questions on forums is not a guaranteed way to get good advice. There are plenty of times I have seen threads on forums where people have given factually inaccurate and even dangerous advice, albeit well-meaning.
In my opinion, (and it is just that) there are many choices with WFP that simply come down to personal preference. For example, some time ago I spent a day working with Paul C Smith who uses Omnipole poles. (Made from aluminium) I use fibre-glass poles. I tried his omnipoles, he tried the fibre glass. Now for me, I prefered the fibre glass becasue the clamps are easier to use. Paul, though wasn't much bothered about the clamps, and preferred the omnipoles because the sections are narrower, and therefore easier to grip.
Now both are very good poles and serve the purpose, but which one is the best for you really does depend on personal preference. The only way that you'll know which you like better is to try them out, and demos will allow you to do that.
Even a question like RO versus DI in a hard-water area. Some might think that's a no-brainer. Ask advice on a forum which you should go for, and everyone will say RO. But this is not necessarily the best option, even for hard water.
I know of a window cleaner who chose a simple DI system even though living in a hard water area because he didn't like the idea of it having to fill up overnight. He couldn't care less about it using more resin, for him this was more important.
-Philip