When the writing task is the primary barrier to learning or demonstrating knowledge, then accommodations, modifications, and remediation for these problems may be in order.
There are sound academic reasons for students to write extensively.
However, for some students writing is a laborious exercise in frustration that does none of those things. One may labor with organizing the concepts and expressing them, learning a lot from the 'ordeal.' The other will force words together, perhaps with greater effort (perhaps less if the language and information has not been processed), with none of the benefits either to developing writing skills or organizing and expressing knowledge.
They may learn much less from an assignment because they must focus on writing mechanics instead of content.
After spending more time on an assignment than their peers, these students understand the material less.
Not surprisingly, belief in their ability to learn suffers.
*Skills or information taught with steps over a series of days, without a preliminary overview or "Big Picture", can be very difficult for Dyslexic students to follow and comprehend.
The constant memory loss of information covered over several days is one way a Dyslexic can be labeled as having "short term memory loss".