In academic writing, there is a tendency for students to write what comes straight out of their head without pausing to think about grammar. While this is fine when one is writing against the clock, it is good practice to use parallelism, or parallel construction, not only at the whole text level (introduction, supporting arguments, opposing arguments, conclusion), but also at the paragraph level. This is not marked down in TOEFL or IELTS but it is looked at in GMAT.
An example of generally acceptable academic writing without parallelism:
The data displays a sharp increase in activity in the summer of each year. Other than summertime, the activity fluctuates and has no overall trend.
The data displays a sharp increase in activity in the summer of each year. Other than the summer, the data displays fluctuations which have no overall trend.
Parallelism looks less interesting but it is easier to read and understand, which is very important in academic texts.
I have an ebook about academic writing coming very soon.