Sorry Paul but you really do surprise me (by the way, maybe we should start a new thread as this has very little to do with NaD).
I'm sure most people have equipment insurance and PLI these days but you seem to forget the majority of incidents happen when people are inebriated - an unfortunate side of our job. You must have witnessed p***ed up people staggering around and if they knock a speaker over it probably wasn't intentional so I think there is no reason why this shouldn't be covered by the client.
Also, you say that if something can be knocked/damaged by children or harm themselves to move it!!! Not always possible, where would you move a speaker or DJ booth if the set-up area was a small area on the same level or next to the dance floor (a lot of venues are like this). Great if you have a stage / raised area but not possible in most venues.
I have only ever done one disco without a contract as it was a next day booking for his daughters 13th birthday (original DJ cancelled - you're not far from me are you
) and it was before the days of echo-sign so I told him the full amount would be payable at the start of the disco and he needed to sign the contract at the same time. Luckily it all worked out ok and he was more than happy to sign (and yes, he read it all while I started setting up).
I have never had a client not sign a contract due to any specific clause as I tell them, it's to protect themselves as much as myself / business.
Let's say you turn up to an unknown venue and the access points are up a narrow fire escape (yes, I've had this), through a narrow door, down a corridor up another 2 flights of stairs then across a large hall right in the corner... then you find that the power points are at the other side of the room? If this had not been mentioned and you were caught in a traffic jam on the way (unforeseen) and you started an hour late - what then? My contract covers me for all this as why should I take a cut in wages for issues beyond my control. It's still a full nights work.
There's so many things that can happen at an event and 99% of the time things run without a hitch, but there's always that one gig where everything goes wrong (we've probably all been there)
Every other professional industry (especially if providing a service) have terms & conditions, why should we be any different?
As for no deposit or non-refundable booking fee - try and book a holiday and say you'll pay for the flight when you're sat on the plane? I think you would get some funny looks...