i agree with maham's post.
as for myself, i'm a "CBCD" and sometimes my mom tells people that my bros are not that fluent in Urdu. She is not trying to brag; she is just stating it as a matter of fact because some people think they're are being rude when they answer queries in English.
I think some newly arrived Pakistanis do tend to look down a lot on their foreign-born counterparts, due to their weaker Urdu skills. i remember one aunty told my mom that "mein to arham se sirf urdu mein baat karti hoon. mein nahi chahti k in ka (my bros) wala haal hojaye.."...anyway that was such a *****y comment...ok now the HILARIOUS thing is that the kid is like 6 years old, and even though the mom only speaks Urdu with him, he replies mostly in English. When he does speak in Urdu, he struggles with some vocab terms....and this is a kid who didn't even know English a year ago.
My point is that a child's brain is very malleable, which is great because they acquire skills at a very accelerated rate, but they lose them just as readily as well. Spending 12 years in a school system will definitely make the school language the primary language, despite any counterefforts.
Speaking urdu everyday at home is good for reinforcing grammar, but the main difficulty is vocab. i can speak perfectly about household matters but if someone asks me to explain math or physics in urdu, i would not have the vocab. also i wouldn't have the ability to write a poem either; these type of skills only come through practice and exposure.
Another thing....in America, they make a huge deal in school about multiculturalism, being tolerant, etc. So if some dorky dude with a huge accent comes to school, ppl will talk slower, listen patiently, offer to be his lab partner, etc. Yeah there's always some jerks who'll make apu jokes but the general thing is to be nice. I've found the same thing of Spanish speakers...I know I make mistakes, but they just smile at me and let me go on.
In Pakistan, it is soo different. my younger brother tried to say something in Urdu and since he couldn't say 'kha' sound** they made so much fun of him and started laughing//and this wasn't a one-time thing either.** perhaps since Pakistani people have less interaction with different cultures they find errors more amusing. but still, i find it very rude to mock a new language learner. it makes retention of even elementary skills difficult.
People do that to me as well :( Saves me a lot of embarassment if I just reply in English..