I agree with @Gilles of course: random italic and bold text is bad. Reject edits diligently, please :-). On a more general note:
For some reason even the markdown help speaks of italic and bold. However, the Stack Exchange engine knows better, and translates this nicely to
Emphasis is meant to make something stand out in a post, thus improving readability of the post. Removing- and especially adding emphasis to someone else's post seems hard, but is in many cases very reasonable... and in many it's not. I trust those with enough rep and powers to (dis)approve edits, using their (in this case probably often subjective) judgement wisely to reject or approve on a case by case basis.
Suggestion: Having said that, it would be nice if the markdown help mentions that italics and bold translate to certain levels of emphasis, and are meant for just that: emphasis.