Presenting for the first time on this blog, my CD collection:
My answer (buried in the comments) was 'Never', along with most of the respondees. I was intrigued by one answer though (comment 33 if you're into scrolling) that admitted "Generally i'm reasonably proud of most of my collection but I have hidden the odd U2 album (actually most of them seeing as I own them all) for fear of being seen as generic."
This touches on a common theme I notice amongst music fans, that the mainstream is somehow a bad thing, commonly expressed as 'I liked (insert artist here) when they were new, but after (insert best selling album here) they went downhill and just write generic mainstream rubbish'. The more famous you get, the less cool and credible you are, almost to the point that mainstream success it seems is frowned upon by serious fans. I've noted that one local alternative radio station often plays certain tracks months ahead of more mainstream competitors, but ceases playing them once the mainstream stations get in on the act.
Admittedly, some artists do make a tidy living by releasing unchallenging mass market sounds. But this doesn't necessarily mean it is bad music. Overplayed and over exposed perhaps, but not intrinsically rubbish. Some of it is intrinsically rubbish, but shockingly enough, some artists achieve massive success by virtue of being really really good.
I have a lot of U2 albums in my collection (all of them apart from the new one in fact). Admittedly the last above average one is 1996's 'Pop', and the last great one is 1991's 'Achtung Baby', but the thing is they are there because I like them, and if you want to stand in front of my collection and mock me for it, thats your problem not mine. A lot of people hate U2, but I get the feeling thats more for exposure and hipness reasons than musical ones. They are definitely uncool, and admittedly my preference is for their early and middle material rather than their latter stuff which doesn't really grab me. But a lot of dislike gets directed at the likes of U2 and Coldplay simply because they are U2 and Coldplay.
I think this is turning more into a rant against precious sneering pretentious music fans than a discussion of whether or not the mainstream is bad. It got to the point a little while ago where I was framing some of my CD buying decisions by the criteria of "Is this cool? Can I have it on my CD shelf? Does it have enough alternative credibility", rather than "Do I like this and will I listen to it?". One of the benefits of getting older is that you care less and less about what people think of you, which means the latter criteria now guides my purchasing.
I would never hide any of my CD's (even if I have more reason to now the collection has been melded with my wife's). I have a lot of mainstream stuff. I have a lot of alternative stuff. I can name a relatively obscure alternative band (The Afghan Whigs) as one of my favourite artists if required.
There is a certain elitism to being a serious music fan I think. What annoys me though is when that elitism becomes simple snobbery. The sneerers would do well to remember though, without a mainstream, there would be no alternative.