It is indeed not something that will make the user to get rich overnight however the rounding issues are giving player an edge over 30 % and that is indeed a big deal .Also that considering your site has an investment feature than it means investor is getting -EV which serves as a bad reputation for your site.

Your action of being hostile towards someone who give you a heads up also disgraced dooglus' calculation about this issue also is a bad publicity for your site and now people will think twice about investing on your site. In other words you have dug your own grave and it will be just a matter of time before more people read this thread

P.S : are you going to give Substrata the bounty for finding this flaw of your site?