Most of the mobile user’s population votes for the unlocked mobiles as they are operated on the user choice. All Unlocked mobile have sim cards, sim cards operate as a medium between the user and the carrier. The best thing for sim card is that if anything goes wrong with your mobile you can easily transfer your sim to new mobile. And this is only possible if you have unlocked mobile. Whenever you get bored with your network services you can easily change your network by having Sim card of other network and enjoy the taste of varieties.
The main issue with the locked mobile is that it is difficult to switch your network to a new one without buying a whole new mobile set with a new sim. The other thing about locked mobile is that the older phone becomes useless for any other carrier. And if the mobile set is expensive then you need to stick with your service provider and it will never give you a chance to adopt any change in the carriers. Users like unlocked mobile due to its less issues. (article take from http://www.articlealley.com/article_2019433_45.html)