I posted this in the General Theravada section, because it doesn't meet the guidelines for the Classical section. In the Classical Theravada section Chris posted a link to a BPS Wheel booklet that has relevance for some recent discussions about the Commentaries (see below). The italics in the text below are mine.
Excerpted from Buddhist Commentarial Literature by L.R. Goonesekere (Wheel No. 113 / 114)
[a few examples of disagreements between the Commentaries and the Suttas]
In the course of the development of the Sinhala a.t.thakathaa, certain deviations from the canonical literature are to be noticed and these are repeated in the Paali commentaries.  Firstly, there were instances where the a.t.thakathaa contained readings different from the text, though in some instances the differences were very slight. Such instances are found in the Jaataka.t.thakathaa.  There are marked differences, however, in the Buddhava.msa-a.t.thakathaa, which contains some stanzas not found in the text and omits others found in the text.  These instances are so many… There are also instances where the commentatorial view differs from that of the text. … Certain terms, too, acquired in the commentaries distinct derivations unknown to the canon. 
i For instance, the term nibbaana which in the canon is connected with nibbaati (S II 85; Sn. p. 235) and nibbuta (M I 487) and has the significance of 'blowing out,' is in the commentaries explained as ni + vaana, `absence of craving’ (S-a I 196: III 112; It-a I 164)
[on the great value of the Commentaries]
The contributions made by the commentaries are both to the letter and the meaning of the scriptures. Variant readings of the Paali texts have been recorded in the commentaries and the meaning of words is established either by definition or by synonyms or kindred and related terms which circumscribe the respective range of meanings. This proves helpful, for instance, with such words and terms the meaning of which in the Paali language differs from Sanskrit usage. The high degree of exegetical reliability of the commentaries is largely based on a perfect mastery of the canonical texts commanded by those ancient commentators. This enabled them to take into consideration all the different contexts in which the respective terms or doctrinal passages occur. Shades of meaning of words or terms are illustrated by quotations from the canonical texts; also where doctrinal statements in the commented text are concerned, their full significance is sometimes strikingly illuminated by the quotation of a kindred text in the commentary. Such widening and illumination of significance is also achieved by another feature of the commentarial method: the commen¬taries often express in terms of Abhidhamma categories what in the commented texts is stated in the conventional language of the Suttas. This also serves to illustrate the doctrinal coherence of Sutta and Abhidhamma.
In the Suttas, there are a few texts and textual passages which would remain largely unintelligible without the commentarial explanations. One typical example is the first Discourse of the Majjhima Nikaaya, the Muulapariyaaya Sutta, of which so far no entirely satisfactory translation exists, due to the fact that the translators did not make use, or not full use, of the commentarial explanations to that difficult text.
The commentarial literature also contains large sections giving full directions for the practice of the several subjects of meditation (kamma.t.thaana), which in the Suttas are explained only very briefly...
P.S.: For the record I revere the Commentaries, but I revere the suttas more. I believe that the Dhamma taught by the Buddha, which was agreed upon by all early Indian schools, is found in the suttas. I believe that I am generally traditional in my understanding of Theravada. (Also, I would no longer attempt to connect this issue with the Kalama Sutta. My interpretation of the sutta has changed. I still believe in thinking it all through and relating it to my practice of Theravada, rather than passively receiving the tradition, however.)