Any kind of coercion in voting, or undue influence during the voting process, is inimical to our individual liberty. The point is that all mail-in voting would eliminate the last monitored and equally accessible polling places, where poll watchers and (mostly) non-partisan volunteers see that voting is done fairly and accurately.  In its place would spring up all kinds of potentially coercive or at least unduly influential situations in which voting would occur, without monitoring of any kind.  There is also the 'lost in the mail,' the 'signature didn't match' and the spoiled ballot without chance to even know it...circumstances inherent in that system.