First- i agree that you just shouldn't use an expensive bike for transportation purposes. Second- consider that if you're car-free you'll probably want a bike that you don't mind getting rain and road-grit on in bad weather.
Both of those factors say you should get a cheap bike of some kind, whether or not you get a fancy road bike. All-weather transportation bicycling wears out expensive (or cheap) bike parts fairly quickly and puts expensive bike parts at risk of being stripped from the bike. You can use an expensive bike but you might wind up spending almost as much as the cost of a car keeping the bike maintained/insured. As far as weekend road rides, a better bike would definitely make a difference in speed, but the importance of the bike is dwarfed by the importance of the "engine"-- but i guess you probably know that.
Commercial culture says that "fun" depends on having something that only money can buy- a $1000 bike, a $80,000 car, whatever. Whether or not you agree with that assessment is up to you.