The only real benefit of adding one at time of purchase is ability to roll it into the loan. You can add these same plans anytime before the 3yr/36k warranty expires, and through the dealers mentioned In threads here, Likley way cheaper than your purchasing dealer.
That said, getting a plan is really a no brainer. Should you decide to sell or trade it in, you can get a prorated refund of the remaining plan. If the buyer of your truck sees the value in it, it's an easy process to transfer
You can get a 5yr100k plan for $1300, one power steering issue and it's paid for itself