This often repeated claim is actually a very good insight into how the Muslims saying it view their religion as a kind of political imperialism more than a benign spiritual path.
if you're generous you put it down to an inferiority complex caused by various factors - some might say its born of a very ridiculous sense of Islam's importance and manifest destiny as the dominator of mankind, hence the volatile nature of the claim - a mixture of extreme superiority complex which leads to a sense of inferiority and cognitive dissonance caused by the difference between the big claims made for Islam, and the reality.
There is also no difference between the claims they make, and the claims made about Islam by various Jihadwatch types. Which means, Muslims who employ this triumphalist kind of rhetoric not only feed Jihadwatch-ism, they perpetuate the view that Islam is not reconcilable with other non Muslim societies and cultures, because it asserts itself as locked into a supremacist competition seeking to replace and dominate them.
Wah, wah, I love reading your posts. They so accurately capture the reality.