Mobile device retailers like Apple, Best Buy, Amazon & Verizon want you to recycle or trade-in your mobile devices. They even offer a good chunk of change to do so.
It’s usually not a terrible deal, but there is one big reason why you should keep your retired mobile device:
Mobile devices break.
They are portable, small, handled often, and as a result are easy to drop.
The screens are prone to glitches and wear-out over the course of a year or two.
Replacement
When they die, you may have a warranty, and your retailer will get you a refurbished one, but it will take a few days to a few weeks.
During that time you will either have to pay for a loaner (which is usually a shitty Droid Eris or something equally crappy), or you can use your own old phone.
You will be much better off with your own phone, but you won’t have your old phone if you don’t keep it.
The Geek Whisperer 
Totally agree. There’s a great tutorial on how to turn your retired mobile into hi-tech again at Lifehacker.
Great point Poch. There are ways to reuse an old device.
I think this is the Lifehacker article you’re referring to: http://lifehacker.com/5576310/reuse-and-get-creative-with-an-old-iphone