Its not that it's bad or wrong, but no rapper wants to "have to" rap that late into their lives. Artists want to create on their time, not on a deadline.
Joe leaving the group was probably the best career move he could have made because he's doing better now than he ever was in music and probably than each individual member of SH.
Shady didn't really know how to market anyone but Eminem, thus they give everyone the Slim Shady/D12 roll out when its just not who they are. Em is a great artist but not very good on the business end