Thing is... not everyone has an alternative mode of transport readily available. Folk cycle for all sorts of reasons - they like it, they have no car for whatever reason, there is no bus service available... If you live and work in the same urban area, you may well have a choice of transport options open to you, especially if you're 9-5 working hours. But as soon as you start adding in stuff such as non-standard working hours, more rural areas, edge of town, commuting distances, then the transport options rapidly become more limited, and cycling may well be the only reasonable transport option for many a journey - whatever the weather.
Edit to add in a general rant...
One of the things that, to use a phrase, gets right up my goat, is when on the weather forecasts and news, we get the phrase "avoid unnecessary journeys" or "essential journeys only" as if for the vast majority of folk going to work/school and the like is something that's done for the sheer frivolity of it... No, let's not attempt to get to work, after all, in these times of the credit crunch we can all afford to have X days' pay docked for not showing up...
Sorry. Normal service will now be resumed.