Slider concept is actually helpful for proper model presentation for the cases when it is needed (for example, in architectural concepting - to present neat looking model to a client, or in landscape design - to detect cliffs, etc), even if it is not used most of time.
But yes, it is limited to 0-90 degrees and displayes 0-1 scale, which is a limitation in my opinion as well.
0-180 degrees scale (like Autosmooth has) seems to be a more pecise and profitable solution in a way:
0 - all edges are visible
1 - coplanar edges are invisible
180 - all edges are invisible