I only advocate writing in optional space as a way for the admissions committee to see that you, as an applicant, are genuinely interested in them; ie. you took the time to write something specifically geared towards them. I would recommend that if you are not an exceptionally strong candidate, you write something. I don't believe anything can hurt you in this game (ie. extracurriculars, research, optional essays, etc.).
It's all up to you though. If you're not compelled to write something, then don't feel obligated to. Point being, do whatever you're most comfortable with.
I would not advise drawing negative attention to your application though, unless you really do have a legit excuse for "bad grades," etc. You might want to try focusing on something more positive about yourself, or somehow change that negative into a positive by showing you might have overcome some obstables.