Whether you need one or not, at this price (no contract), it’s just one of those things you shouldn’t need to think twice about.
At 19USD, there’s no reason not to get the 521 even if it’s just as a back up phone you’d store away.
I’d get one if you can.
Whilst you’re here, it might be worth checking out Cricket as they’ve got the 630 for $50. No annual contract. There’s a $50 rebate too which makes it free/1 penny ‘profit’. But there is a monthly ‘plan’ so in a way, not really that cheap. So go for the 521 if you have the option. 🙂