Frequently changing battery from mobile Is is bad for a device to change battery frequently from mobile?

I have one extra battery when my first battery got dry I removed it from mobile and put the second charged battery. Is it bad for the device?

I am using Samsung galaxy s2
It is okay to use another battery as a backup. But if you switch the batteries frequently, you may end up with damaging your phone. Even though the chance for this a very low, it is still a chance.

When you change the battery frequently, you have to turn on and off the device. When you do this, the inserted battery spends power to start and turn-off your phone which is more of an unnecessary power waste. As a side effect, your phone covers may be at the risk of loosening, as a result of frequent use. 

Other than that, there is nothing wrong with the use of two batteries with your device.

Instead of changing them frequently, you can switch batteries after using it for a certain period. (e.g. a week) Your backup battery will always be with you holding a full charge for a week. See the battery tag for more info on battery usage tips.