Why would you even consider buying a dual sim mobile device? Having a dual sim phone means that you can keep two separate numbers for messages and calls.
Depending on the device you can even set it to prompt which SIM is used for calls, data services and text messages. Phones that support the Dual SIM service allow you to switch between your SIM cards seamlessly.
Dual SIM phones are very beneficial if you perhaps run a business and would like to have one number for business and one for personal use.
You can also save on your cellphone bills by taking advantage of different peak/off-peak specials and even splitting your voice and data usage.