OK, Will, taking your point regarding the reasoning behind locking the doors whilst travelling first. For many, and I include my wife in this, this facility offers/adds to the feeling of overall security. Several years ago whilst returning home late at night, there was an attempt by several hooded youths to enter my wife's car whilst she was stationary at some traffic lights. Fortunately, with the doors set to automatically lock (VW Touran) upon reaching a set speed, the attempted break-in was unsuccessful and she was able to quickly accelerate away albeit quite shocked of course. This type of incident is not isolated by the way and whilst it's not repeated at each and every traffic light etc it does occur on a daily basis.
Regarding the hammer and webbing cutter - I must admit to raising an eyebrow here. In an earlier life I was a serving Fire Officer for over 23 years with a good percentage of that time spent on operational duties. One station that I served at covered a lengthy section of the M25 and M23 in addition to some very fast and heavily used 'A' roads. In short, I couldn't begin to tell you how many RTA's that I have attended - some slight of course whilst others I wouldn't care to describe on an open forum. Being able to reach a child secured in a back seat assumes of course that (a) you and/or your front seat passenger are uninjured and in a fit state to execute a rescue, (b) that the impact of the accident has not jammed either of the front doors - a frequent scenario and with the additional safety features in today's cars e.g. seat belt tensioners located in the 'B' posts and floor sills - the 'creation of space' as it is termed in the service is an art in itself.
Of course, like many things in life, we are free to make a choice and so I am not attempting to convince you or even convert you to the idea of travelling with locked doors. The choice is yours to make, Will, and whilst I am convinced that the advantages outweigh the disadvantages you might think differently.