It seems that (only) some file associations went wrong. In this case, you could have just right clicked on an skp file, go to Properties and define the path to SU 8 (instead of the removed SU 7).
Whether to leave it or keep it... You can of course keep it, no harm. Especially if you use the free version, keep it as the plugin that was provided for dwg/dxf import for v.7 no longer works in v.8.
Also, georeferencing (GE snapshot) is completely different in v.8 and you may want to keep v.7 to be able to grab a rotated view for instance...