How many people actually shop based on convenience, or whether their can of beans are 10 cents cheaper at another place?
I imagine that if you factor the value of your time, the amount you save is more than offset by the time spent driving and shopping at another store. So, if you earn $30K per year, then after salary, benefits, etc, your time on average is worth $20 per hour. That's a lot of nickels, dimes and quarters for the extra hour required to shop at Giant. (This is presuming that both stores have the same product, and you're shopping and choosing based solely on price.) For me, I go to McKays and another grocery store, because each store has a different preferred brand of food that I buy regularly. If McKays had both, I'd skip Food Lion or Giant when I run out of the other product, and buy everything at the local company.
Of course, grocery shopping can be a hobby for people who don't work and old people, but as someone who fits into neither category, I prefer convenience over price any day of the week.