You are supposed to write in parenthesis when you want to have the effect of whispering additional information to the reader. This is done in situations where:
[*]writing a whole sentence would be run-on, or
[*]writing another sentence for that whisper would ruin the logical flow of the paragraph.
If those conditions apply to you (such as in this example), then you should use parenthesis. If someone is bothered by the parenthesis, that person can read the sentence while skipping all parenthesis. The paragraph will still have a smooth flow and the essential points would still be gained.