I actually think that MOST councillors are retired (though that is made up, but it seems likely). I would also hazard a guess that the vast majority of councillors don't have jobs that require them to be in London most week days.
Finally, most people's jobs allow them flexibility in the way they work. No one else can vote in Parliament when John Leech is in the City Council chambers. It's not like you can just get a temp in. He can't do both jobs at once because of the nature of the jobs, so naturally he ends up doing them both slightly less well than he could otherwise have done.
It is also a disgrace that along side him representing me badly as my councillor and as my MP, that he then takes two lots of my money to fund his political office.