I use secondary basic phone to accommodate my extra SIM card. As my career is IT, I will be placed in different locations. So I was forced to buy a secondary phone to accommodate extra SIM card. Every time I get a new location I need to buy a new SIM card to avoid roaming. Sometimes I might give different mobile number to different banks. So to receive update and One Time Password from bank, I need to have all the SIM cards with me. I want my primary phone to be capable of all the task like Wi-Fi, multi-tasking, email etc. So I have a good smartphone as my primary mobile. My secondary mobile should have double-SIM capability and able to recieve message.
If battery backup is key problem, that person can opt for battery bank. Battery bank will give extra 12 - 24 hours of battery power depending on the product you buy.