That's never been the case for me. This is my 6th new phone with T-Mobile in 8 years and they have never sent me a new SIM card. The only time I got a new SIM card was when I moved from Michigan to Illinois and changed my number so it would be local, then I had to get a new SIM because they're locked to regions or something. They couldn't just change the number, the SIM card had to be changed, too.
Other than that, though, I've had the same card. When I had my phone number changed they said they could not send me a new SIM because they do not have them, that the stores have the SIM cards. That's what I had to do, drop by a store and pick up my new card.