Perhaps the one drawback is that you have to front the cash for the second device at the register, and then wait for a rebate. Verizon typically turns rebates around pretty quickly, so you shouldn't have to wait too long to be reimbursed.
This would be a perfect way to equip a number of employees with some low-cost smartphones if you have the need.
The deal runs until October 10, so you have plenty of time to take advantage of the offer. My guess is, Verizon will have a bunch of new phones to show off starting some time after October 10. By then, the holiday shopping season will be approaching quickly, and Verizon will want new phones on the store shelves.