I am one of those very few (now you can all die happy knowing that I'm intact), but the most common reason I hear is hygiene. Though I have no freakin idea what the hell that has to do with this form of "whacking" it off.
Due to a research project I have done on AIDS and HIV, I read that the part that is removed during circumcision is more susceptible to the HIV virus than the rest. And having it removed slightly lowers your chances of catching the virus. But even if your partner doesn't have this virus (or the latter disease), you should still use a condom unless you want to lose half your paycheck for the next 18 years.
I don't know why it should be a religious practice either, I don't think saying "cut your johnson off and you'll get to heaven" is a very good selling point for anything, much less a religion (yes I know it isn't all removed).