Benefits of using 3G with dataCard instead of mobile phone:
1. Save battery - When you use 3G on mobile, its battery discharge very fast. Compared to 2G, 3G takes more energy. and when you use internet mobile display work continuously compared to when you talk on them.
2. Airtel 3G connection switch to 2G sometimes and it deduct from main account which cost much. I think they have solved this. But I have noticed it twice in past.