This should only be an issue if exactly one major party does it. If the other major party says, "Give our three candidates 5 stars each, everyone else 0", it should counter the instructions from the minority party. In practice, I expect this behavior would be very common in swing districts; the difference between this and Approval-style fusion (in which each party can endorse any number of candidates on the ballot, but without suggested scores) would come from minor parties that do use more expressive endorsements. And in less competitive districts, an endorsement along the lines of "give the opposing-party candidate who's most willing to compromise 1 star" makes a lot of sense. In any case, parties can endorse in a way that prevents the other side from gaining a significant advantage, and voters should have more information than they would with more basic forms of fusion voting.