Well theres a whole world of philosophy and art embracing or at at least exploring such thoughts so it must matter to some people. Perhaps person3 (if i may be so bold) is alluding to the possibility that these thoughts dont necessarily have to impinge on the structure of a persons life, that its possible to live with them and even remain externally unaffected (in which case others at least would detect no change).
It sounds cliched and easier to type than to live but, the only thing to fear is fear itself. Its definitely related to DP (and an integral part i guess).
This is really the DP flipside of the DR 'is this real?' question.
'Am i real? Is this real? Its all an Illusion!' That is what derealis(z)ing and depersonalis(z)ing are all about (in the formulation of thoughts that is) for any thinking person. All it shows is that you are responding to the abnormal way you percieve things in the way any sane individual would.
If you are questioning yourself then who is doing the questioning? As long as there is a 'questioner' capable of reasoning then it seems likely you are doing ok and are not in danger of evaporating mentally/spiritually. I know this fact doesnt really help me a lot of the time but its important to remember that its just a part of DP and the bad times pass and things come together again at least for a while.
Noones life is perfect, you just have to take what you can get and help who you can help i suppose.