I usually find that the people who carry both an iphone and another phone around do it because they have to have the other phone (better deal, or company phone) but then have an iphone because they wanted it but only ended using it as essentialy an ipod touch but with a 3G connection, to which I say why not have just got an ipod touch.
sometimes you need 2 numbers (therefore 2 phones) but I havent come across many that actually needed two
sometimes you need 2 numbers (therefore 2 phones) but I havent come across many that actually needed two