What could be done with a deactivated SIM card?
The SIM card contains the IMSI (International Mobile Subscriber Identity) number and the associated private key. The IMEI (International Mobile Equipment Identity) is part of the phone.

Basically, the IMSI is the user name, and the private key is used similar to a password here. You can reuse that SIM card only when you find an operator willing to accept your account data.

Commercial operators just give you a new SIM card, as it is way cheaper to give you an envelope than to read the IMSI off your card and reactivate it.

What you can still do is read (and write) the phone book and SMS store on the card.