Rick’s not eligible for about 10 of the delegates in IL IIRC. It’s a fair bet most of those are in more liberal counties he wasn’t going to defeat Mitt in anyway. He doesn’t need Newt in the race there. The VA strategy of Newt/Rick types voting Paul is sufficient there and offers Rick his best chance in the rest of the state.
Just because Newt pulls out doesn’t mean you can’t vote for him over Paul. Some people have still been voting Perry or Cain. But Newt pulling out would certainly give Romney wins in those districts against Paul as they did in VA.
IL is essentially winner-take-all by district, in the form of direct delegate elections. So it’s a very dangerous vote-splitting scenario. Mitt only needs his delegates to get 26% of the vote in a district to win that district if the others split evenly. If it was Santorum vs. Mitt, and Santorum could get at least a slightly higher vote than Mitt in every district, then Santorum would take every delegate in the state.