Fog lights. Its all in the name.
As others have said, they can dazzle other drivers and unless visibility is less than 100M there is no need for them. In my 5 years of driving I can count the times I have genuinely needed fog lights on one hand.
I don't believe some of you are using the "they're are called fog lights" argument. You do realise that there are more things than fog which can cause reduced visibility, right? Like for instance....heavy rain?
By your logic, should I only use my sidelights when driving sideways?