Have we not done the mirror thing to death before guys
All extra mirrors can be useful if properly placed/positioned etc etc, the more info you can get from ALL round the vehicle has to be a good thing, no matter for what purpose.
When reverse parking if you need to get out, walk round and physically move the nearside mirror then that is what you do, rather take a few minutes and avoid that concrete post
To be honest I fail to see the problem with reversing into a space, a few seconds taken to assess the size, shape etc should be all you need to park with reasonable accuracy, most spaces are a uniform size and 2 or 3ft wider than the average car so as long as you can see the line your side you do not really need to see the nearside line (basic maths should divide 3ft into 2 equal bits).
Always comes back to the same basic point...take your time, look, assess, position the vehicle properly, re-assess if needed....park
But then of course as it has been pointed out on several occasions I am only an "experienced" driver....I hate those stupid "" things