One case where a single gigabit port might help.
If you have gigabit capable 'server' connected to the gigabit port, and three 'clients' connected to the other ports.
Each client can only talk to the server at 100Mb. However, in theory, they can all talk to the server at 100Mb each simultaneously.
(I don't know if the HomeHub or other single gigabit port router would actually allow it all to go that fast or not).
Or, if there is a gigabit switch involved, the three slow 'clients' can talk simultaneously to any device on the switch at 100Mb each simultaneously.
--
Moved (with trepidation) to BT Infinity 2 for upload speed. Happy BE user for several years.