If the CMRI encourages people to vote and the SSPX encourages people not to vote, then the CMRI is right on this issue and the SSPX is wrong. One can abstain from voting if all the candidates are unworthy (it doesn't matter if one is more or less unworthy), but that is for a particular office, not an entire ballot. It can't be terribly common that there are no good candidates at all on the whole ballot.