Limitations to training on singlespeed?

What's the reasoning behind fixed training in the winter as you understand it clubman?

From what I gather its to keep the pedalling technique in top form, promote the suppleness in the legs before trying harder efforts to get in form for the season.

I'm not completely sure I fully understand the reasons. Two minor reasons are:

Tradition. For some people the mere fact that something has been done decade in,decade out is enough to suggest that there may be something in it. Of course, there are many who take the opposite view: if something's been done the same way for a long time, it must be out of date.

Economy. Perhaps not a factor today, but we used to think it was best to keep our more fragile geared bikes in the shed (or perhaps the bedroom) until the severe weather and the salt had gone.

I think there are probably two good reasons apart from these. Firstly, as others have said, there is the strength with suppleness argument - this would apply at any time of year, but if you're going to ride fixed for only part of the year then winter seems the obvious time to do it. Secondly, winter conditions lend themselves to long, steady distance riding - and this has generally been the style of early season training runs. If you're setting out, in a group, to do,say 60-90 miles at a steady 17-20 mph then a single gear in the 65 - 70" range should be perfectly adequate from a speed point of view, and at the same time it should avoid the draining effect of trying to push too big a gear at too early a stage in your programme.