I agree with you, to a large extent.  A plurality of identities is generally a good thing.  If you go back a bit further into history I think it's true to say that most people most of the time do enjoy a plurality of identities and it is only in times of crisis - of actual or perceived oppression - that single identities come furiously to the fore.  At times that can be quite a positive thing.  It was an important part of the NUWM's fight in the 1930s that large numbers of radicalised unemployed workers saw themselves first and foremost as unemployed workers and that was their key identity.  Similar things could be said about locked-out miners in '26 and international brigaders in '36.  The extent to which a single identity comes to the fore can be seen most starkly by the nature of funerals in those situations: members of the NUWM who died were usually buried amongst comrades to the singing of socialist anthems.  It is unusual, bizarre, but helped galvanise support for campaigns - a whole culture was built up around them.

When we look at the sort of religious/ethnic identities you refer to, in 2006, a whole array of actual and perceived attack is there.  The most obvious example is with the Muslim community in Britain today, and is what Shahid Malik and others were talking about in their letter.  They very clearly were not talking about the purpose and motivations of the invisible 'masterminds' of terrorism, but about what makes normal working and middle-class young men and women become so subsumed into one identity that they would use themselves as a weapon to kill their fellow men and women.  And British foreign policy - along with a lot of home policies for that matter - inevitable are a factor in that process, and quite a key factor.  I've taught a lot of young muslims, and they - like everyone else - have the myriad of identities you refer to: they're Yorkshire men and women, they're working class (or other classes), they're Labour voters, or Liberal Democrats, they're dance music fans, they're indie kids, they're chavs, etc, etc.  But more and more in recent years (and obviously partly because I teach them Politics and Sociology so we'll talk about these issues more than others might) there is a consciousness about what is happening to Muslims around the world, and more and more it is what the British government are doing (often what they're actually doing, sometimes what they're perceived or reported to be doing) that dominates these discussions.  For the most part I'm encouraged by the politicisation - it's constructive, it's no more 'radical' than my own politicisation at the same age and I've never planted a bomb (well only a very small one!) and frankly if it leads to a few more votes for the Lib Dems or Respect then that's the democratic process, and it's probably no more than we should expect.  The people debating these things at college probably aren't the people you need to worry about.  They're hearing about it in a political context and are learning about political processes and solutions.  

I was going to get onto other national and religious/ethnic identities but I've babbled on too much.  Basically I was agreeing with you, but wanting to introduce another element to this: the impact and responsibility of government policy.