The problem is that if your candidate preference is A > B >> C and you vote for A and B (and not C), you might cause B to win, even though you preferred A. The tactic for just voting for candidate A is called "bullet voting", according to [1]. An interesting analysis [2] of the 2007 French presidential election claims that Approval Voting was the voting system that was second least vulnerable to strategic voters (behind the authors' own voting system, of course :)

[1] https://en.wikipedia.org/wiki/Approval_voting#Strategic_voti...

[2] https://en.wikipedia.org/wiki/Tactical_voting#Approval_votin...