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. 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.
Writing is a complex task that takes years of practice to develop.
Effective writing helps people remember, organize, and process information. However, for some students writing is a laborious exercise in frustration that does none of those things. Two students can labor over the same assignment. It is important to stress that the issue is not that the student can't learn the material or do the work, but that the writing problems may be interfering with learning instead of helping. From this discussion, everyone involved can build a plan of modifications, accommodations, and remediations that will engage the student in reaching his best potential.