If people in a "full" train can be persuaded to move down the car a
little to allow more passengers to board, then there is a net benefit to
passengers on that line of holding the train. If you mean literally
full, i.e. crush-loaded with absolutely no room for anyone else and
people waiting on the platform for the next train, then I agree that
ideally the train should depart and be held at the next station if the
crush-loading has eased. But you would need a more flexible signalling
system than currently exists, where AFAIK only certain signals can be
held on red in this way and the rest are automatic. There is still
probably a net benefit in holding the full train, though.
A very good explanation I'd say. Plus at stations further down the
line if passengers see a full train with a long gap behind they are
probably more likely to try and shove on the train with possibly even
more delays as doors have to be shut several times, abusing staff if
they then can't get on etc. And, building on the point above, on most
lines that run through central London (rather than just to it like the
Met) the train is unlikely to be totally packed throughout its trip so
regulation somewhere is sensible (and I'm sure most people would say
that's fine so long as its after "my" stop)

P