Bike Training (increasing efficiency) question

It keeps you from getting sick.  Biking is more forgiving than running, but even on the bike, you can't always train at race pace, or you'll either not get in enough volume, or you'll injure yourself or get sick, or all three.

But you DO need to do SOME training at or even above race pace (depending on the length of your race).  Most plans that I've seen make this clear.

In any case, since you are new, the most important thing is to focus on volume, I think. 

Training at a lower intensity allows you to traing with a much higher volume, a key to increasing your aerobic engine.  If you trained at race pace, your volume would have to be much lower.  With a lower volume, you wouldn't be able to get as much training adaptations.

However, as the previoud poster stated, some of your training needs to be at race pace, and a smaller percentage needs to be faster than race pace.  The majority however, will be slower.