In the UK, putting 13A BS1363 sockets on a lighting circuit is frowned upon (although not explicitly prohibited), but 2A or 5A BS546 outlets are often put on lighting circuits to allow control of free-standing lamps from the room's light switches.
While the contacts are separating, any electrical energy stored in the inductance of the circuit being disconnected is dissipated as an arc within the switch, prolonging the transition and worsening the heating effect on the contacts.