How does voting for someone else other than Mitt Romney help Mitt Romney?
Currently, Romney is getting about 25% of the Republican vote in the primaries and in most of the polls. That means 75% of Republicans are voting for Not-Romney.
If the rest of the vote is divided more or less evenly between Newt, Santorum, Perry and Ron Paul then the those candidate will end up with less than 20% and will lose to Romney.
If Perry drops out then some, but not all, of those votes will go to other mostly non-Romney candidates. If this doesn't happen then the liberal Romney will be the one to represent the mostly conservative Republican party in November.