Ellen G. White and "The Bible and the Bible Only"

Do Seventh-day Adventists believe that the writings of Ellen G. White are equal to, or an addition to, the Scriptures? If the Bible is all-sufficient, why do we need Ellen White's writings?
Seventh-day Adventists do not place Ellen White's writings on the same level as Scripture. "The Holy Scriptures stand alone, the unique standard by which her and all other writings must be judged and to which they must be subject" (Seventh-day Adventists Believe . . . , p. 227). Another way of framing this question is to ask why the church should need any of the promised gifts of the Holy Spirit. Ellen White answered this question in the Introduction to her book The Great Controversy Between Christ and Satan:
"In His word, God has committed to men the knowledge necessary for salvation. The Holy Scriptures are to be accepted as an authoritative, infallible revelation of His will. They are the standard of character, the revealer of doctrines, and the test of experience. "Every scripture inspired of God is also profitable for teaching, for reproof, for correction, for instruction which is in righteousness; that the man of God may be complete, furnished completely unto every good work" (2 Timothy 3:16, 17, R.V.).
"Yet the fact that God has revealed His will to men through His Word, has not rendered needless the continued presence and guiding of the Holy Spirit. On the contrary, the Spirit was promised by our Saviour, to open the Word to His servants, to illuminate and apply its teachings. And since it was the Spirit of God that inspired the Bible, it is impossible that the teaching of the Spirit should ever be contrary to that of the Word.
"The Spirit was not given--nor can it ever be bestowed--to supersede the Bible; for the Scriptures explicitly state that the Word of God is the standard by which all teaching and experience must be tested. . . .
"In harmony with the Word of God, His Spirit was to continue its work throughout the period of the gospel dispensation. During the ages while the Scriptures of both the Old and the New Testament were being given, the Holy Spirit did not cease to communicate light to individual minds, apart from the revelations to be embodied in the Sacred Canon. The Bible itself relates how, through the Holy Spirit, men received warning, reproof, counsel, and instruction, in matters in no way relating to the giving of the Scriptures. And mention is made of prophets in different ages, of whose utterances nothing is recorded. In like manner, after the close of the canon of the Scripture, the Holy Spirit was still to continue its work, to enlighten, warn, and comfort the children of God" (The Great Controversy, pp. vii, viii).