Let's be open-minded...
I think a lot of this debate is down to the fact that we like to be part of a group who are 'officially' diagnosed with the same thing because we want to know for sure that their experiences are in some way similar to ours, or at least as severe as ours.
I know people with many DSM diagnoses who are very 'protective' about their diagnoses. I have bipolar disorder, and have been admitted to hospital 8 times, sectioned 4 times, had 6 courses of electroconvulsive therapy and so much medication that I have been unable to walk or talk. I have nearly lost my life to this illness several times. So when someone says to me 'oh yeah, I think I'm bipolar, I get mood swings too', I admit, I get pissed off because people don't seem to realise how serious the condition is. It is a lot more than being a bit moody. And bipolar is another condition which seems to be very 'trendy' right now, and probably over-diagnosed or misdiagnosed. So I know that I can be a bit defensive over this diagnosis and often doubt that other people actually have this condition even though they think that they do, just because I feel that their symptoms are less severe than mine or whatever. I know I shouldn't think like this, but sometimes I do, and I find the same thing with people with other diagnoses too.
But one thing I will say is that a lot of symptoms of different disorders do overlap, hence the big 'categorical versus dimensional' debate over the DSM which is going on right now. For example, schizophrenia and bipolar disorder have the highest rate of being mis-diagnosed as one instead of the other of any psychiatric illnesses, because so many symptoms of these illnesses can overlap, although many people don't realise this. For this reason, similar/identical psychological and drug therapies can be helpful for these two different conditions. Thus some advice can be helpful even for people with a different disorder if some of their symptoms are the same. Not always, but sometimes. So I think it is important to accept this and not exclude people or judge people because they don't have the all important 'diagnosis' that oneself has, although of course, it is always a good idea to get professional help rather than self-diagnose.
The other issue is that naturally, some people will have a more or less severe 'version' of an illness than the next person, and it is also important to bear in mind that some countries or health trusts are much more likely to diagnose particular psychiatric illnesses than others....although we have a DSM, it is not adhered to religiously. Research has consistently shown that many people can go to two different psychiatrists, say exactly the same thing, and come out with two different diagnoses. Often this can depend on the facilities that a place has for dealing with a particular diagnosis, and all sorts of other underlying issues. I have seen this first hand through my PhD studies in clinical psychology.
So I say, let's be open-minded. Yes, for a person's own sake they should seek out a correct diagnosis, but we shouldn't exclude people or 'look down on them' if they don't have the same diagnosis as us. It is not a competition.
As you may have noticed, I have pretty strong feelings on this one, so excuse me if that sounds like a slight rant!!!
KB.