Like anything, the practice of clear cutting in forests has both advantages and disadvantages. Especially with a topic as controversial as this one, it is wise to look at both sides of the issue. There are indeed some major disadvantages of harvesting timber by clear cutting it.
Not all tree species are the same. This may sound like it is simplistic, and it really is, but it has a huge bearing on logging practices. Some trees, most notably firs, grow quickly. Though this is a subjective term, it means that in about four or five decades, a fir tree can grow from a seed to maturity. What is more, a fir forest can benefit from controlled clear cutting, since the young trees thrive in full sunlight.
The same thing can't be said of most other trees that are commonly harvested. For instance, pines seldom grow so close together that the seedlings won't get enough sunlight. At the same time, they require more time to become old enough to be harvested. Often, even in a high yield forest, a Ponderosa pine will be 65 to 75 years old before it should be properly harvested. That is with the best of conditions for growth.
Hardwoods like oak, maple and hickory take substantially longer to become adult and full grown trees. Many oaks that are 300 years old could be thought of as just passing their adolescent years. Clear cutting these trees leaves a mar on the land that takes a very long time to heal. It is also unnecessary, since the young trees usually have plenty of sunlight for growth. They will usually do best if the area is selectively logged.
Land degradation and erosion
Many people, even when walking through a forest, don't think about one of the most important aspects the trees provide. They take care of the soil. Living tree roots hold the dirt down, preventing a lot of the erosion that would otherwise occur, while also providing material, like leaves and needles, that break down to provide nutrients for other growth. Clear cutting removes the living roots to hold the ground together, and the means of providing leaves that can break down for further growth.
The planet is resilient, and will overcome these obstacles; however the act of removing all or most of the trees in an area means that it will take longer for the forest to recover. Erosion also takes a huge tole in a clear cut area, as winds and rains carry the most fertile soil away.
Bad logging techniques
While federal and state forestry departments do monitor logging practices within their jurisdiction, the simple fact is that most forested land in the US is privately owned. Too often, the desire to make some money overtakes the common sense to do it properly. It is most cost effective to clear cut. If it isn't done correctly, the result can be severe damage to the land and especially riparian habitat. This harms every plant that grows there naturally, as well as the many animal species that rely on those plants.
There are many laws that are strict and rigorous, in this regard. However, there simply isn't the manpower necessary to enforce them on a small scale. Even when infractions are found and pursued, the damage is usually already done.
Except in isolated situations, such as a dense fir forest, clear cutting usually has more disadvantages than it has advantages. People might wonder why it is still done in other places and situations. Chances are that the answer revolves around money and lack of consideration for the environment.