It's simply a later regulation - one reason is the introduction of plastic pipes, previously all pipes were copper, so all were nicely connected to each other. Now, some may be copper, and some plastic, so it's possible that some sections of copper pipe could be lacking an earth connection.
As for 'dangerous', like all earthing schemes they decrease some possible dangers, but increase others - if your's isn't bonded, then there's no need to do it unless you want to.
I've recently had new central heating (last November) and I don't think they earth bonded everything? (I'll have to look), but they used all copper piping, no plastic at all.