I'm half in and half out of the ianrauk way of thinking in that I rode my best bike all year round regardless of the weather and 9 years later despite minimal TLC is still polishes-up to near showroom condition (if I can be arsed).
However, for winter I put on heavier/stronger wheels, fatter tyres (25c) and raised the bars a tad to account for more slippy conditions, less visibility of potholes and having a slightly more upright position for visibility and comfort when wearing extra layers.
This year I will have a Winter/Audax bike and a Summer bike and the winter bike is the more expensive, not cheaper. It'll be more comfortable, have dynamo lighting, fatter tyres (28c), more upright position, more durable wheelset, full mudguards and some light load-lugging capability for long days in variable conditions. The Summer bike is old, but will have lightweight wheels and tyres and minimal other stuff.
Then it'll be simply which bike for which day/ride/conditions. On bright winter days I'll take the 'Summer' bike out, on anything else it'll be the 'winter' bike.
Of course there's also ... the folding shopping bike, the fixed, the pub bike and a few other projects as yet un-built ....