The most important reason to pay attention to parallelism and other aspects of writing like word usage and length of sentences is that you want your audience to understand you. You also want to persuade them to believe that you are an authority to speak on that subject and are intelligent and educated. Your audience can sometimes understand faulty parallelism, but it often makes the sentence more difficult to read. In fact, you will frequently find you need to re-read a sentence with faulty parallelism to understand what the author is saying. Moreover, writing correctly in parallel structure shows that the writer has excellent English writing skills and can write in a professional and highly proficient way. I often edit the work of University professors who have written many papers. The only consistent error I often discover in the writing of these highly educated people is faulty parallelism. Therefore, if you can master this difficult grammar rule, you will prove your competency.