Background noise clashing with speech is a commonly cited reason for turning subtitles on, even without hearing impairments.

Control over sound levels is often impossible in dynamic situations, but in scripted conversations, cutscenes etc, bear in mind that what sounds like good balancing through your own setup will not necessarily be how players hear it, due to either differences in their hearing or differences in their sound hardware, so err on the side of caution and ensure that important audio such as speech is as distinct as possible from any background audio.