R beats D.
The primary is the chance to choose the best R. But if the good R managed to blow the primary, then it's still better to vote for the crappy R. Because the alternative is a D.
This matters more in legislative races than executive. In a legislative context, you must consider the value of overall control of the body in addition to the value of the individual candidate.
E.g., if worthless Kirk caucuses with the Republicans, that's one vote closer to control of the Senate and one vote closer to Cruz or Lee actually having their bills voted upon, as opposed to being buried by Dingy Harry.
Having to vote on popular Republican bills embarrasses Democrats, who have to choose between nasty campaign commercials and abandoning their President. Having Congress actually pass popular Republican bills forces Zero to veto them. So, sure, they don't become law. But at least the Won had to react. Which he doesn't, now. Thanks to Dingy Harry and 'Rat control of the Senate.
Ain’t no “R” in this race. Rauner is a leftist Democrat using the Republican line. The only real contest would be if an anti-Combiner got the nomination, and that happened in the 2010 Governor’s race (which was promptly stolen by the Combine for Quinn, just as they fixed it for Kirk, another leftist Combiner). It makes no difference which one wins this time, because the people lose (as well as good government).