I can't honestly say I truly know the impact of the 'brain drain' on Pakistan. I think Indians, Sri Lankans, BD and many other countries' 'youth' are going to the richer Middle Eastern and Western countries. Just a few days ago I saw a docu in which was claimed '1 out of 7 Irish people want to leave Ireland'. And from following the blogs, it appears a great desire to emigrate is present in many countries--or at least that seem exaggerated online.
In case of Pakistan, with a huge population where the universities and technical schools churn out hundreds of thousands into the labor pool every year, I suspect Pakistan can afford the 'brain drain'. It is not like some countries with small population whose 1/4th of the population are expats! I think Armenia is one of them, if memory serves me right? With lack of enough opportunities domestically, overpopulated countries might be benefiting from the export of labor. That's my guess. Again, I don't know for sure.