I have a couple extra SIM cards for the same number, should I keep?

As from my understanding, when the program a SIM card with a number any other SIM card gets deactivated in their system (only one number can exisit). Each SIM has it's own unique ID and why they can diable a phone off their network by telling their system that ID is no longer valid.

My guess is that those SIM's won't work on anyphone now unless one gets reactivated by the carrier and thus the one you have will stop.

The SIMs are not associated with the phone number used on the current SIM. Depending on the state of the SIMs on the network they may be usable for connecting another service with a new number, or replacing the current SIM with the same or new number. These activities require provisioning by the phone company to the network. This can only occur if the SIMs are still registered (but not active) on the network. If the SIMs are no longer registered then the you might as well throw them out. Most phone companies only register SIMs to the network when the receive them from the manufacturer.

It would probably be good to keep them as backup because they easily wear out i swap my sim card from my iphone to a blackberry about once a month and it has already worn out once. AT&T charges $30 for a new SIM card so it would be a good idea to hang onto them because the cards also can randomly stop working sometimes. You also can keep them to give to a friend who needs one because they can asign a number to a previously used one

Well...they are handy if you have a table that's a little wobbly - else, no.

Only one of them should be active with your number. If you put in a different one does your phone still work?