SB, I used to be the same way, but I've been doing dayhikes and mountain training the past year as much in low-top approach shoes as in boots. Definitely lighter. I have noticed a tendency for my ankles to roll a bit more on rugged terrain, which is only natural without the high-top protection, but with a light pack and poles it doesn't necessarily concern me.
Before making a call on trail runners for an extended backpack, though, I'd load up the heavy pack for at least a day's hike in them on rough ground and see how it feels.
