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.
If the race drops to an essentially two man race - Mitt vs. the anointed not-Mitt, plus a Paul technicality, and IL is winner take all by district, then the goal is to avoid Mitt district wins where possible. There may be some Mitt majority districts. We can't fix them, whether or not Rick is eligible, by strategy, only by conversions.
Anti-Mitt strategy can only affect non-majority Mitt districts. There, when the anointed not-Mitt is eligible (as in 44/54 IL delegates), that means only one not-Mitt available on the ballot. Leaving 'rejected not-Mitts,' with hard core fans, available, risks letting Mitt win some districts he could have lost. As really happened in MI and OH. (And for extra hard core Newt fans who claim Rick fans should have done the same when they were ahead, only in FL was that alleged to have been an issue; there Mitt beat Newt plus Rick's totals, so really Rick never cost Newt any state.) If any of the Rick ineligible districts (10/54 IL delegates) are non-majority Mitt strategy can't give Rick a win, at best it might deny Mitt one on a technicality. Actively put the message out for Rick voters to vote Paul whenever Rick's not on the ballot. Those complying, plus the 5-10% Paul true believer vote, might cost Mitt a couple delegates. Telling Rick voters to vote Newt misses the true Paul votes and risks a Mitt plurality win. True, a Newt delegate might be better after the first round convention voting, but first you have to get beyond the first round. An anti-Mitt Paul first round vote is easier to strategize than an anti-Mitt Newt one here. VA was the proof of concept.