Just because other people do it, doesn't mean you should. If people jumped off a bridge, should you too?
Tap water contains salts, calcium, and other minerals. When that stuff gets boiled, they leave scales on any surface it can settle on. When that stuff builds up in the radiator, it will clog it up, which reduces cooling efficiency. Once it gets bad enough, you will have problems.
This is why the informed people use demineralised water in their cooling system as there's nothing to cause problems in the water itself! Manufacturers have always been aware of this problem and so the trend is to do away with any antifreeze concentrates sold at dealerships but instead sell premixed jugs of coolant. Haynes manuals are also a very poor resource for those who want accurate information too. They are written for the laymen who don't know anything about cars or want to learn much about them except for simple basic maintenance. It has been my experience also that they have buggered up their repair procedures for certain things for certain cars as well as specifications. This is to be expected since they basically just copy information out of the real manuals that manufacturers publish but goof up doing so in the process (it's easy to do this when you got so much information to copy then rewrite). Nowhere in the factory's shop manual does it say to flush the cooling system, although usually nothing is said about using demineralised water either.
Both of those pages linked also are from North America, which compared to most parts of Europe have much softer water (except for certain areas which are served by well water) which in majority of cases causes no problems at all except for reducing the antifreeze's life. Many Japanese cars from the early 2000s or older uses the traditional antifreezes, but under a Japanese formulation which uses phosphates as the corrosion inhibitor (everyone has now switched to hybrid organics which does away with phosphates/silicates/borates). When a hard water is mixed with a phosphate based coolant, major build up occurring is not unexpected. This has never been an issue here in North America or Japan, but due to such an issue and the common public who never consider what's really in water ("if you can't see it, nothing else is there" mentality), European manufacturers were using borates as the corrosion inhibitor instead which does not have adverse issues with hard water.
Like I said, if your cooling system has always been well maintained using the proper methods, then a flush is completely unnecessary. If the coolant is all rusty brown, dark, etc-- anything that isn't green or dark green, then by all means suspect that the system has been neglected and there is rust/buildup inside.
My own car has 90000 miles on it now, the engine block still looks like new inside. Not a speck of rust inside the cooling passages when I took off the water pump at 60k. Just grey clean cast iron!
What comes out of the radiator is also green and not a speck of dirt or rust. I always changed the coolant under the recommended schedule in the manual and always used distilled water to mix with the antifreeze. So anyone else who has been doing the same thing should not even have to worry about flushing or their cooling system.
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