The dual SIM card cellphones are very popular abroad. They are great for travel. Normally when we travel we make arrangements with our carrier to implement a temporary data plan with a certain amount of data usage. This usually comes with a $1/ per minute cost to place a call. Most people will usually restrict communication to text messaging and emails and finding wifi hotspots when away from your hotel in order to place calls using skype, facetime or some other voip protocol.
The dual SIM card allows you to simply pick up a SIM card from a local carrier in your travel destination, activate it with the amount of usage that you will need and insert into the 2nd SIM slot. Once the phone recognizes the 2nd card it will prompt you to choose which SIM you want to use to send a message or make a call. You can make either SIM(number) your default.
By simply texting your friends and family your temporary contact information they can stay in contact with you and you control how much it will cost you. Any incoming call can be responded to with a text informing the caller to use your temporary phone number to reach you. You can call home and conduct business as usual saving hundreds of dollars.
An example: while in Jamaica with a large group. We negotiated a rate with the local carrier (LIME) for 200 minutes for less than $20US. This included a deposable phone. With the dual SIM card phone I was able to use my existing phone, insert the new SIM card and use my phone to communicate while I was there. With my US carrier I would have paid a fee to activate an international package with 100MB of data and $1 / per min for calls. This would have been over $200US. By having a dual SIM card phone I was able to save over 180US
The savings are even more when traveling throughout Asia, South America and Europe.