Short Explanation: Keep it short!
This is less a flaw in the reasoning process than a flaw in the argument or deliberation process. Just because too many words have been expended on explaining an idea or position doesn't mean that there is either anything wrong with the conclusion or with the process which led a person to that conclusion. It is, however, a barrier to communicating these ideas to others.
Naturally, it is the communication of ideas which is the point of debate, argument, and discussion; therefore, anything which aids in communication should be treated as valuable, and anything which inhibits communication should be treated as a problem. Communication may not be the sole factor when it comes to evaluating an explanation, but it is an important factor.
Why does excess verbiage occur? There are a variety of possible reasons and not all of them are bad. One very understandable reason is simply that we write in a manner which is similar to what we read we imitate, even if unconsciously. People who read simple things are likely to have a smaller vocabulary and end up writing simpler things. People who tend to read very complex and difficult material will have a larger vocabulary and can end up writing things in a more complex manner.
This is not a bad thing on the contrary, it shows that in order to be better writers, we need to spend more time reading better material. However, people who do read difficult texts need to be aware of how that influences their writing. When their audience is also accustomed to such texts, then there probably isn't a problem; on the other hand, when their audience is accustomed to simpler material, they need to pay closer attention to their writing and make sure that others can understand it.
There are other reasons for excess verbiage which are less acceptable. Some people may simply be trying to impress others with their vocabulary and writing skills (of course, by writing in such a manner they are actually displaying a lack of said skills). Others may be writing in a very bombastic style because they themselves are very pompous and full of themselves, not realizing that their manner of writing makes getting at the ideas more difficult than is necessary (or just not caring because their purpose in writing doesn't include communication).
The use of excess verbiage is not so much a flaw in reasoning but a flaw in the argument process because of the way it inhibits communication and is a barrier to a proper evaluation of a person's ideas. Nevertheless, because such a style makes it difficult for others to understand what a person is saying, it is reasonable to wonder if perhaps it is also a sign that the author herself fails to understand what she is saying.
Although it cannot be assumed that one always leads to the other, it is true that an incoherent presentation of ideas is often a sign of incoherent thinking and an inadequate understanding of the ideas involved. People who have a very good grasp of what they are explaining are usually able to present their material in a clear and coherent manner. In order to determine whether this is the case instead of some other reason (like those described above), simply tell the person that it is difficult to get through their explanation, ask them to simplify it, and see what happens.