Essentially because it is better to use it than not. That said, there is a long standing tradition within poser to "simply do it my way" -- which means not working within the constraints of IK. IK exists because it allows the figures to better mimic actual human movement, and it ultimately makes it easier to create poses (in particular, animated poses). However, even some of the biggest names in in the Poser world never got a handle on using it, and it annoyed them, so they started turning it off. Others started turning it off, and so forth, and so on. so, less verbose, it's on becuase if you learn to pose with it, it makes life easy. Learning to pose with it is a pain in the butt, though. So turn it off ;)

thou and I, my friend, can, in the most flunkey world, make, each of us, one non-flunkey, one hero, if we like: that will be two heroes to begin with. (Carlyle)