I think that something else that can weigh in on a decision to spend a bit more than you might be able to pay off immediately are the emotional benefits, which may be worth the interest costs. If you have good credit and have a good rate, then it might be even less of an issue.
For instance, if you want a new subwoofer and you've got $500, but the new one is going to cost $1000 and your interest rate is good, perhaps the enjoyment you gain from having the sub now and for the next 5 months is worth however much you'll be paying in interest on the $500 you can't pay off now.
Another example would be a family vacation. You take your family and kids to disneyworld (or somewhere nice) for a vacation that will take a few paychecks to pay off, so you say to yourself that the enjoyment everyone has from that is worth a little extra money to make happen.
I'm not advocating that people spend more than they have the money for, or to count on money they don't already have, but I can definitely see where a little overspending can be worth it. I'm guilty of both situations. A pair of speakers for the first and a rental car for the second. I love the speakers and use them everyday, and convertible mustang rental was more than worth it while cruising around Florida
YMMV