By the number of replies to this post you could say "not much".
I think this is a wrong conclusion and a bit too vague. Look at how many registered users opensubtitles.org has (over 2 million) and how little are actually active on this forum (more or less only a handful). Many users don't know about this forum. From those who do, many only come for one specific question or problem they have. So, no reaction on this topic doesn't mean so much.
Could the administrators give us some figures?
According to the WHO, 10% of the world population have some hearing impairment, varying from mild to total deafness. From somewhere halfway that scale, somebody would be depending on HI subtitles to be able to fully follow a movie. Let's say from 'moderately severe' - this is approximately 1,5% of the world population. Roughly 80% of people with a severe hearing loss are 60+ years old. Percentages vary per country.
I think there are no opensubtitles.org specific statistics. Even the number/percentage of downloaded HI subtitles wouldn't say a lot, because maybe that's the only one available, or it's better than others (and used to remove the HI additions), or maybe they are only used for translation, etc.
But my personal non-scientific guess is that most users of (open)subtitles are younger than 60. Given this, and the above global figures, the percentage of users depending HI subs would be less than 1%.
Is it worth the effort to make these subtitles?
I would say "worth the effort" does not necessarily depend on numbers. Maybe it's a choice between making a lot of people a bit happy or making some people VERY happy. And from some comments here and there I conclude hearing impaired people are very happy with it.
Besides, and I suppose you are talking English subs for English movies, those subs can be used for translations into other languages, with or without the HI additions.
So yeah, I think it's worth the effort.