The expensive bike won't rust. The expensive bike can be repaired when things break. The expensive bike is half the weight.
If you are using your bike frequently, it's better in the long run to have decent hardware.
Bikes mass is marginal compared to your body weight especially if you're average weight. A basic bike is 34 lbs and you are 175 lbs. And the weight only makes a big difference when climbing hills.
You're much better off losing a few pounds of fat than spending a few grand to get 10 lbs off the bike and shaving 2% off the required muscle power per mile :)
For most people the weight makes the biggest difference when climbing stairs (with the bike on your shoulder).
Beyond mass, the behavior of shifters and brakes on nice bikes is much, much crisper and more predictable than on cheaper models. They also have better capacity for tuning (e.g., to eliminate chain rub, or accommodate a high-range sprocket set), and are more likely to stay in proper adjustment for longer. This makes riding a lot more fun -- the machine is very responsive to your wishes, and you forget it's there.
You probably are aware of this, but I felt a need to mention some of the advantages of a nice bike.
However, this doesn't apply to racers. If it's going to speed someone up by 2%, and he's got the money, and he wants that last tiny advantage, he may just drop the cash on a $1500 wheel set.