I agree with the rationale that you prefer, which is voting based on the candidate's stated governing principles, with an eye toward being able to persuade people to accept that package of policies. I think what Palin was trying to convey was to avoid a rush to conclusion based on the few primaries already conducted. If one candidate wins all the primaries, then the outcome of the process is clear, and people lose interest in the message - "the electable" one has been chosen. But we don;t have an environment where one candidate has won all the primaries.
I am not sure what she means by "level the playing field," unless that is a reference to an unfair or inaccurate public impression left by one or another of the campaigns. In this race, all that would boil down to as her caution against adopting Romney by dint of the media (and pundits, and openly purchased political ads) presentation. The presentation is lopsided, and it'll take time for that to sink in to a gullible and fickle public.
Well, play it out in your mind. What would be the ultimate end result of a level primary playing field?