The message isn't, "Don't read", and it certainly isn't, "You may only read when you've trained x years and achieved y rank." The message, as I see it, is that when you are a beginner, reading isn't going to help you, and may in fact only serve to confuse or mislead you.
There are a couple of aspects of "beginner" that are relevant here, IMO.
The aspect of being a beginner that I think Mary was focused on is spelled out very clearly in her next post.
To me, being a "thinker" on the mat is like relying on physical strength in learning Aikido -- the individual has come to believe that a certain quality they perceive in themselves is an asset in dealing with life situations, and habitually rely on that quality even when it does not serve them well.
It can sound equally paradoxical when a thoughtful or physically strong person begins training in Aikido and hears "don't rely on your understanding," or "don't rely on your strength." I think the apparent paradox stems from the unrealized need to break from old patterns. (Relevant disclosure: I still hear both messages myself).
Just as practice ought to offer a different and more supple kind of "strength" over time, so too ought it offer a different, arguably more supple and immediate, way to "understand." And just as the strong person repeatedly will discover their pre-existing way of using "strength" is a handicap, the intellectualizing person repeatedly will find themselves stuck in a perceptual well and trying to react to a moment that already has, for practical purposes, swept on by.
Another aspect of being a beginner applies to the student "of" aikido as an intellectual subject. We all start learning the corpus of what has been written about Aikido from an arbitrary point, and our partial knowledge may well be different and/or appear more certain than it would from the vantage of having read (or practiced) for a longer time (preferably, I believe with the kind of ratio of practice-to-thought that Mary described).
Compared to many who post here, I consider myself still a beginner on both levels on most issues. As for the value of these forums, one for me lies in a wealth of detailed information provided by those who have spent many years studying and practicing. Another is a greater appreciation for the multiplicity of legitimate meanings Aikido may have either as a practice or as an intellectual subject.
That said, I'm afraid my ratio of practice-to-thought just went askew....