It's not good for passengers if most people board the train in the middle at most stations. That just means the middle carriages are likely to end up much busier than those at the ends.
This is why the arrangements on platform 2 at Kingston seem so wrong.
There is always a tendency for regular passengers for a particular station to favour the carriage which will stop nearest the exit from the platform at their destination to try and beat the rush through the barrier line there (watch how the rear carriages of Up trains empty out at Wimbledon for example) This is particularly so for Kingston, where the barrier line is quite inadequate for the rush-hour crowds (it is the busiest station on the Loop but the only one to have only one exit - SWT and SWR have resolutely refused to re-open the other one, for no obvious reason, but ton the great annoyance of their regular customers, most of whom live on that side of the line)
A ten car train stops with the ninth car nearest the stairs. 8-car trains now stop at the same board as ten car trains, so the rear of such trains is about one carriage-length
beyond the exit from the platform. The exacerbates the overcrowding in the rear carriage, which is always the most crowded in any train leaving Waterloo because of the very short notice given of departures there*.
It also means that the rear of the train is next to the narrowest part of the platform, causing everyone to have to squeeze through the bottleneck before reaching the wider part - and of course there is a four foot drop off the platform behind the train which people can be crowded into. Also a hazard for visually impaired people waiting for the train, who may have an expectation that the train will stop opposite the entrance to the platform rather than go right past them and stop further along the platform, leaving them unknowingly facing not a train door but a four foot drop off the platform edge
Latecomers trying to catch the train cannot see it as they come up the stairs because the buildings are in the way, and stand no chance of catching it anyway in the face of hordes of people coming the other way through the aforementioned bottleneck.
And the dispatchers' office is also beyond the back of such trains, making their job harder as they can't see whether people have been able to board (and have a habit of giving the "right away" when prospective passengers are still "letting the passengers off first" - not acceptable on a 2 tph service)
And of course only four carriages are under the awning, instead of six.
I have not had a satisfactory explanation as to why the old "8 car stop" sign was removed. Apparently it was "to avoid confusing drivers". But they seem to be expected to manage on Platform 3 (where an *8 car stop" sign is still to be seen). And I don't recall any incidents of 10 car trains stopping short because the driver thought it was an 8 car.
The problem will become less when most trains are 701s, but there will may still be occasions when 450s appear.
(*there was a missed opportunity, when the platforms were extended as far as Westminster Bridge Road, to have a second entrance to the station at that end - which would probably have had support from MP's and DfT staff living in the South West as it would shorten their journeys to work!)_
