There is a peculiarly British slant to all this, which stems from the history of cycling in this country, and one which is expressed even in mountain biking.
Wearing continental trade team tops in the days when all 'proper' cyclists rode for clubs marked you out as a loner, a newbie, and not part of the gang. There are different levels to the problem though; it's seen as acceptable, even fashionable, to wear kit from some obscure Belgian bike shop team; I saw someone wearing a hat from Plum Vanquers the other day, yet wearing a Sky jersey suggests to some that you might not be that knowledgeable.
It's the same as buying a Specialized bike; you get a sense that some might look down on you because it's not made by a man in a shed in BC.
It's like using Mavic factory wheels instead of Open Pro/Ultegra made by some greasy aproned artisan who once applied chamois cream for the brother of a man who was a semi pro.
It's part of being a cyclist in the UK, and it's funny that people take it so seriously. Ultimately it's similar to that 'I've been into them for years' thing about bands; as soon as everyone likes them, the cognoscenti drop them like a stone.
I've been droppped by a man in a parka wearing trainers while on a chaingang; its all good..