Dividing the language is bad in general, since it essentially divides the ecosystem behind them. In the case of Perl 6 the ecosystem couldn't control the ambitious development goal; in the case of Python 3 the ecosystem could control that but could not realize that the smooth transition plan is still a transition. There are less problematic but similar causes in other languages, PHP 4 vs. PHP 5 vs. PHP 5.3 (oops!), D 1.0 vs. D 2.0 and so on.