I was confused and concerned about this too.
I must admit, I'm not a fan of this, even if it's setup in a way such that a backport can never supersede a official packages. Especially as this is an LTS, I see two issues with this:
Bad branding, bad first experience - I think almost any user who first comes across this tab will think, "Oh no, scary unsupported updates are somehow enabled on my super stable Ubuntu 12.04 LTS!"
The default is to let users unknowingly install unsupported software of possibly poor quality - I doubt it's clear in the Software Center whether a package is only in backports or not. We want the user to trust the Software Center. The default settings should have a strong social contract: "Everything you install from here is high-quality and trustworthy". Of course, a user might add PPAs, and then the contract has changed. But the default should be safe and reassuring.
I'm a Debian greybeard, and when I saw this, I immediately thought it was a bug. I unchecked it, and then used synaptic to make sure nothing was installed from backports.
Honestly, I'd like the backports checkbox to be removed. PPAs are a much better, fined-grained way for someone to install unsupported software, should they want to.