I think much of the time why modellers prefer B&W to colour is because their white balance is so far out, so converting that shot to monochrome gets around that issue. We've all done it I'm sure, especially if trying to shoot something at a show with all the mixed lighting casting different colours on everything, or if you're a bloke, simply not reading those new camera instructions to work out how to set the white balance!
Tip: shoot RAW and write your white balance later at home if your camera allows!
Very occasionally I do get that shot which gets more B&W hits than colour, this is probably something to do with the lighting and composition. Those shots tend to have a strong graphic quality, any colour adding little or no value. Luckily these lazy days we can make that choice in the virtual darkroom at a later stage. But if shooting a B&W shot specifically, it's always good to 'see' in B&W at the taking stage, concentrating on those aspects that make a B&W shot work by ignoring the colour aspect. Google Colin Gifford or Bill Brandt and you'll see what I mean.
|Tide out at Le Touquet - originally a colour shot, but the|
blue channels darkened and yellow channels lightened during
B&W conversion to replicate the effect of B&W film shot
through a red filter. CLICK TO ENLARGE!
The most popular filter was yellow, the yellow filter brightening yellows and green in a landscape (or the yellow end to a loco) and darkening the blues, great for a summer days when wanting to separate the fluffy white clouds from the blue sky. For more extreme effects orange or even red filters could be used, red in particular darkening the blue sky to almost black and lightening the yellow/red end of the spectrum to make these colours appear white in monochrome. For railway photographers this could give the most most bazaar effect to a BR Blue loco on a sunny summer day!
To celebrate B&W in miniature, I have created a Flickr collection of some of my favourite B&W shots - they can be found here.