We have a LittleLife backpack too. DD wasn't a bolter, never has been - more of a "THAT LADY IS SO NAUGHTY, SHE CROSSED WHEN THE MAN WAS RED!" shouter... 
..but that backpack has still been used a fair bit. For instance, walking back from the supermarket, crossing a very busy A-road, carrying several bags, no free hands - I could hold the strap in my hand with the bag handles, loop it back and give DD her own strap to hold for support and to keep her close and upright if she stumbled. It gave her freedom to walk rather than being strapped into a buggy all the time.
It was also useful in shopping centres - DD would be very unlikely to run off, but she's terrified of losing me and found the backpack reassuring. Mostly she carried the strap in the bag, but if it was really crowded, she'd ask me to hold onto the strap so she knew where I was
.
If I had a bolter, though (and I probably do now - 15m DS is showing every sign of being one), I'd have no hesitation in imposing reins/backpack wherever it was necessary. DD's now 3.9 and offered up her backpack yesterday, because "DS keeps wanting to run away and we don't want him to get run over, do we?".
I'm all in favour of giving a child as much independence as they can handle - but only in safe conditions.