I thought about making an app like that as well, except for android, but never got around to it. I felt it would be useful (for me) to quickly check for new posts to certain threads, and the posts could be formatted better (easier for me to read) then my browser does.
I say go for it, even if no one else uses it, if it helps you it's worth the effort.
And, unless you haven't already though of this, I think that that it should have notifications for when someone replies to one of your topics or a topic that you have participated in, that is able to be turned off, because it might get annoying.