Sarge has a point that if they aren't causing injuries, then it might be fine to wear them.
But while the forces when walking compared to running are much lower, you typically wear the shoes much longer (10-14 hours/day vs 0.5-1 hour/day).
The reason you replace running shoes is often because the midsole (and probably outersole) are permanently damaged. The damage in this cause means the little air bubbles in the material are ruptured and they loose their stability. Worse, the damage might be asymmetrical, because the material wasn't 100% uniform to begin with (they might look the same, but they probably aren't). This means that during every step you take on damaged shoes, means you subject yourself potential asymmetrical loads, which can lead to injuries.
This also goes for your regular shoes, though these tend to be made from different (hopefully tougher) materials. Though with the trend of people wearing running shoes as their regular shoes means they probably wear them for much longer than conventional wisdom would advise.
So your choice is: do you want to risk getting starting symptoms of injuries so you can wear these shoes a bit longer? To answer that question, its a good idea to look back and see if you're susceptible for injuries and if so, what helped prevent them. If you're walking pattern is very symmetrical and you hardly ever have injuries, go for it, chances are you won't get injured. If you're like me and get injured every year, don't risk it, because its not worth it.