what is "WM"?
the point about installing and learning how to use and configure sphinx is just that. you have to download, have the access needed to actually install it,
learn how to configure it, etc. etc.
until and unless your board gets really huge, there is no benefit to using it. It wouldn't hurt anything but you would also not get the benefits of having it.
and, once again, wp is not a discussion board . you can allow comments to each post you make but it is not really a disucssion back and forth.
they are two different things.
wp and phpbb have nothing to do with each other, they are different programs developed completely different organizations for completely different purposes.
having said that, you can certainly install them both at the same time on your website.
that will allow you to create a blog about whatever you want and then have a robust discussion board/forum for your blog viewers and you to discuss the topics you post about in your blog.