1. Some students' papers are filled with simple sentences. Simple sentences are sentences that contain not much more than one independent clause.
1.' Some students' papers are filled with simple sentences, i.e. sentences that contain not much more one independent clause.

2. Simple sentences are sentences that contain not much more than one independent clause. Simple sentences are fine for the drafting process. Strings of simple sentences create a "choppy" feel to the writing.
2.' Simple sentences are sentences that contain not much more than one independent clause, and while simple sentences are fine for the drafting process, strings of simple sentences create a "choppy" feel to the writing.

3. Strings of simple sentences create a "choppy" feel to the writing. Revising simple sentences requires combining sentences.
3.' Strings of simple sentences create a "choppy" feel to the writing, a feel that can be avoided by combining sentences.