First, some allow commentaries to replace their own careful study of the text. Rather, commentaries should serve as a dialogue partner between the student and the text. It is never a replacement.
A second problem is that commentaries are often used like a dictionary. Some look up comments on passages like one looks up a definition of a word, a word here and a word there. The problem is that most commentaries do not trace the flow of a text's thought in its individual comments. To understand the commentator's understanding of the flow of the book's argument, one will often have to read more than just the specific comments on that passage. Better yet is to read the entire commentary itself.
Now I realize that many in ministry are pressed for time and on occasion one might resort to the misuses above. However, one should avoid its habitual practice. It is like eating a candy bar in place of a well-rounded meal. One might do it on occasion to get by but doing it as a matter of practice is unhealthy.
By the way, these thoughts were spurred on by William G.T. Shedd's remarks from his commentary on Romans. Not the italicized sections that I added.
"In short, the endeavor of the author has been, to furnish the theological student with an aid to his own conscientious examination of the original text of the Epistle to the Romans, and thereby to the formation of an independent judgment and opinion which he will be ready to announce and maintain. It will be reward enough, if this commentary shall be the means of stimulating any to the close and lifelong study of the most important document in the New Testament, after the Gospels. Demosthenes read Thucydides over and over, seven times, for the sake of forming that concise and energetic style which has been the admiration and the despair of orators. Whoever reads St. Paul's Epistle to the Romans over and over, not seven times only, but seventy times seven, will feel an influence as distinct and definite as that of a Leyden jar. But the study of St. Paul, like that of the speeches in Thucydides, must be patient analysis. The great characteristic of this Epistle is the closeness of the reasoning. The line of remark is a concatenation like that of chain-armor, of which each link hooks directly into the next, without intervening matter. The process of an exegete must, consequently, be somewhat similar to that by which a blind man gets a knowledge of a chain. He must do it by the sense of touch. He must handle each link separately, and actually feel the point of contact with the preceding link, and the succeeding."
William G. T. Shedd, A Critical and Doctrinal Commentary on the Epistle of St. Paul to the Romans (New York: Charles Scribner’s Sons, 1879), vii-viii.