Finer soil caused by over-tilling becomes compacted and is not able to hold enough oxygen or water. Good soil should contain about 25% air, 25% water, and 50% soil particles and organic matter. Compacted soil can be helped by adding mulch to help the soil absorb water and promote the growth of beneficial organisms in the soil by increasing and maintaining a higher soil temperature. You can also prevent soil from becoming impacted by spreading gravel over the areas in your garden that are used as pathways for machinery. The gravel helps to distribute the weight of the machinery and lessens the potential for impaction. Tilling impacted soil is certainly not the answer and may even be nearly impossible depending upon the severity.
Certain areas of the garden, however, can benefit from frequent tilling. If you are trying to control a pesky plant problem, such as poison ivy or kudzu, frequent tilling can disrupt seedlings and starve existing weeds of nutrients.
You can develop a strategy to reconcile these two ways of tilling. Areas of your garden where planting occurs should really only be tilled twice a year - before and after harvest. Other areas, such as pathways, can be tilled more frequently. You can also combat the effects of over-tilling by being sure to add plenty of organic matter to your garden. Compost that has not been completely broken down can also help add texture back into overly-tilled soil.