The main reason I prefer a lighter bike is for when I need to carry it up some stairs or something like that. For most everyday riding I don't think weight matters much.
I feel the same about carrying the bikes up stairs :-)
However there is a noticeable difference in riding as well, unless you ride only on short distances.
You can simulate the difference on the same bicycle so the comparison is not tainted by different tires and mechanical components. You can add 5 kg (or 10 pounds) here and there or even in a backpack (but it will harm your performance in other ways). Maybe don't put weight on the steering/front wheel because that will make for a difficult handling. You can use water, so you can release it at any time and feel the difference.
You'll notice that with the extra weight you accelerate less and climb worse. All sort of disadvantages derive from that. If you do 100 km and you have a choice, you'll go for the lighter bicycle. Unfortunately that means a lighter purse as well, maybe much lighter.
