Being able to use a second carrier without cancelling a contract with the first is useful for international travel; the 'roaming' rate is often much higher than

Being able to use a second carrier without cancelling a contract with the first is useful for international travel; the 'roaming' rate is often much higher than the normal rate of other countries' local telcos. Plus, mixing-and-matching plans can be advantageous to the user in certain situations even within one country. For example, data-only plans are typically cheaper than data+calls, but one might still want a very modest calls plan for infrequent use - sometimes, better value can be attained by combining two separate contracts. Of course, whether or not telcos ought to allow such use of their contract handsets is another question entirely!