It is for the visually impaired, and most often som sort of inverted theme. You have it in windows, os x, iOS, android and so on. Many websites has this as an alternative as well. It is simple, the more that are able to read the more people can access the site. Not everyone want's to have everything read to them via screen readers, and crappy code most often mutulates that experience as well.
Images has the alt-tag for example, so that people who don't se images can make out the context anyway or people who have disabled image view in their system.
In the name of usability I belieive that a native hight contrast theme would gain you cred. as a software company.
It will, i believe, also by its existence perhaps enlighten one or two web admins that there are more to it then just how you self perceive the world. But most of all, I don't believe there are many out there who will put time into it, even if it is as simple to use the theme-maker-thingamajig. But if included it could be easily made availble, hence make the web more usable for more people.
It would be highly useful for some of us to be able to connect tags, like synonyms.
For example if you could connect automobile and car, so a search for car as well as automobile would bring up all topics with those tags.
If you are a cars forum, obviously you wouldn't want to connect those word. But for the rest of us, with users that tag according to different standards it would probably be a better user experience with more relevant search results.
There are many words that are used for the same thing according to political landscape, upbringing and social class. And english and american have a couple of those as well... So if you see a trend that tags divide the same type of content, you would be able to connect those tags and the problem would be solved from that perspective at least.
You probably wouldn't save that much money eitherway so just buy it.
I have an inactive license but I would need $200 for that, its forum, blog and gallery. The community suite. And then its $25 more to get it active for 6 months I believe it is. I also have an inactive vBulletin but that is $195 to upgrade to the 4. See the connection ;)