When a full train empties out at a specific station you can get massive delays. Euston platforms 8-11 come to mind. Two arrivals of 600+ people (including standing) trains in a minute or so in say 8 and 11 can cause chaos.
It depends. Usually you'd be right, but for some big events, the stations and platforms can be incredibly packed. In those cases the extra delay from gates could really hurt. One example is Comiket, where you have thousands of attendees all coming to the same few stations around the venue. Both times I was there, there was a massive crowd spanning from the platform to the outside. Having to wait the extra few hundred milliseconds on each card tap would have been painful.
