XD Very true!
But that's allways a problem when speaking of -other- people!
For example, as you said, you can't call arab all the muslim cultures... and neither you can call all these cultures muslim, as there are christian, drusian, pagan, etc, and many other elements in them... and not allways everybody is muslim!
and one can't tell them asiatic, or from a geographical area, as they are spreaded in 3 continents
And if one call wrongly somebody of a culture, they will feel insulted!
Like when I tell the US.American I thinked they where from New Zealand, Canada or Australia (happened me more in facebook than here, anyway XD)