Love, friends and politics - Why shut out those who do not agree with you?

I have noticed a trend recently where people say they would not be friends with, date or marry someone who had differing political beliefs from their own beliefs. One poster went so far as to say she would not return to her dentist after discovering his/her political leanings. I do not dispute that they have the right to do this. But, is it wise? Why or why not? And, what is your political affiliation, if any?