Re: This seems similar to the global snooping (encryption) issue.
"The idea that evidence that was obtained illegally should be deemed magically "invalid" is utterly mad and it amazes me that so many people seem to accept this bizarre idea."
'Evidence' obtained illegally includes things like http://www.cnn.com/2017/07/20/us/baltimore-cop-allegedly-planting-evidence/index.html which was done because the cops _knew_ that the 'bad guys' were guilty. (Another example: the socks in the O.J. case, the ones with blood stains on the _inside_ and with crime lab blood preservatives in the blood... https://www.quora.com/Was-O-J-Simpson-framed-by-the-L-A-P-D) Cops fabricate evidence all the time. Having rules means that they can't always get away with it. (Just _mostly_ get away with it.) You cannot, you simply cannot, have people break the law in order to enforce it and still have a law enforcement system which has public respect. Which is why in large parts of the US and the UK a substantial proportion of the public lack respect for law enforcement. Why, I'm sure that I've seen evidence of such a lack of respect right here on El Reg.
"You want murderers released into the community because an incompetent (or perhaps corrupt) policeman didn't follow the correct procedure"
Yes, I do. Either the law is there for everyone, including cops, or it isn't. And if it isn't, then there will be trouble. If the cops can do anything they like to create or 'find' any evidence they need, then any 'suspect' will _know_ that he's in for it as soon as plod knocks on the door. Which means that, as he's gone anyway, might as well be hung for a sheep as a lamb, and he'll tend to meet plod with a shotgun.