Fighting with open faced helms is really only viable in stage combat, and that includes improvised stage combat. Meaning, you're not doing a real martial art. If you're fighting true to the arts, there are plenty of thrusts, and the face is one of your primary targets. Hence the need to have perf-plates.
Having open faces or bar grills is simply a big risk even if the rules disallow thrusts, because a bad parry is all it takes. So long as the sword tip is smaller than the openings, there's a danger.