I don't think this is a good idea. We can have a rebate/ credit on an account when you activate a new account with a plan for 3 months. This would be better and it gets people to commit for a bit longer while not needing to pay for the SIM card.
I'm all about free stuff, but I also understand the waste involved when something is "free" - usually ends up costing us more down the road. I do really like @RohanRon_ON's point about crediting the SIM card cost to your account after being with PM for a designated period of time (3 months, 6 months, etc). Even though it is a small cost, it would certainly be a nice gesture - much like the rest of the existing rewards system.