not really. i am from karachi and no one in my family wears that. i always thought it was more of a punjabi thing :) . but i guess it depends on your family customs.
It's not a Karachiite custom...my family is Kashmiri, and we use them too. It's actually an Iranian custom, that was carried over into South Asian Shia culture, and then adopted by many Sunnis, especially in areas where there is a lot of Shia influence (UP, Hyderabad)
Imam-e-Zamin was a descendant of the Prophet Mohammed, and one of our imams. It is believed that he blesses travellers, so people traditionally used to carry around coins incribed with his name. The coin was a form of "nazr"..you promise to God that you will donate it to charity if you arrive at your destination safely. Nowadays people call those embroidered arm bands Imam Zamins...they fill them with money and tie them on whenever they start something important...a jouney, a marriage, etc...and then donate the money afterwards.