The review and summary don't quite match
If the Lumia 710 barely makes it through the day and costs £300, why would you consider it for your second phone?
For most people, the use cases for a second phone are:
A) A phone you can rely on to be working when you can't use your 'primary' phone - usually because the battery is dead. Possibly if you need to use a foreign SIM etc.
B) A phone that's so cheap you can take it walking, sailing, canoeing and bungee jumping* without caring too much if you lose or destroy it.
So you'd expect a second phone to have a battery that lasts much longer than your main phone, and be dirt cheap - something like the £10 jobbies from Tesco, or that Energizer-bunny one.
The Lumia 710 doesn't fit either of those use cases, so I really don't understand which demographic you see using this as a second phone.
I can however see the Lumia 710 as being a second choice if you can't afford the smartphone you really want. That's especially true as there's so little reason to pick the Lumia 800 over the 710.
*Ok, maybe not bungee jumping. Skiing?