There is some truth to this - WES also considers scores higher than 60% to be equivalent to a GPA of 4.0 on the 4.0 scale.
Having said this, we have observed that this is a score that could potentially be a low score in some Indian Universities (and high in others) - so a reasonable evaluation would take your university as well. This is the conclusion we've reached based on the data that we've collected at August Academy. I'm fairly certain schools would have quite a bit more data than us - and over a longer time period.
My recommendation, then, would be - if you belong to a university where the scores are traditionally high (Say Anna University, Osmania, etc. Even at the IITs a score fo 8+ merely places you in the top 30%), then write the optional - provided you have a good explanation. If you don't have a good explanation, then the point is moot.
If you belong to a university where you score is good, then don't worry about the optional essay.
Given that you clearly state that your score placed you in the bottom 50%, I would say the former applies in your case.