1. The person says "I don't know" fairly often. It shows that the person is not quick to draw conclusions, based on little to no information; this is gold, it means avoiding a lot of personal drama where they could blame you for things that they assume that you did.
2. They're generally on the same page about common acquaintances as you.
That's different. I'm talking about avoiding to rush towards certainty, not lack of opinion/preference.
That said, "I don't know... it's too late to buy groceries, but we got a frozen lasagna, there are some vegs in the fridge, and I could whip some fried chicken if you want. What do you want?" sounds perfect for me. So the issue here isn't the "I don't know", it's the lack of input.
[Dunno if you were speaking seriously or joking. If joking, sorry for the serious answer.]