Why correct frame size is so important
Choosing the right size frame for your body is not only the first step in obtaining a comfortable and efficient position, but also ensures that your bike handles correctly. Generally, a road bike is not designed to be ultra stable, and behave like a loaded touring bike, and at the other end of the spectrum, neither is it supposed to maneuver like a super responsive track bike. Most road bikes are designed to handle in the middle ground, somewhere in between those two extremes.
Although it is true that a bike's front end steering characteristics are largely defined by its trail (definitely a topic for another day), trying to fix an ill fitting frame that's horizontally too long or too short by using a long or short stem, or by moving the saddle too far fore or aft, can compromise the handling characteristics. What do I mean by "handling characteristics?" Well, the bike can feel like it's fighting you when cornering, or may not want to track straight when you're your climbing out of the saddle, or can feel like it's too responsive (i.e. "twitchy"), or can shimmy when descending.