The combination of sallet and bevor was a very popular head defense for most of the 15th century in spite of an obvious vulnerability. A rising thrust could pass below the lower edge of the sallet, particularly from the side or rear. The wearers were probably mostly worried about frontal attacks: once an opponent has blindsided you or gotten behind you, you are probably in trouble in any case. Still, the sallet and bevor did have a vulnerability that other contemporary alternatives did not. Nonetheless, it was very popular. Why?
It is helpful to understand what I believe is the default position for a sallet in battle. It is very frequently shown in contemporary illustrations with a significant gap between the bevor and the lower edge of the sallet where the wearer’s nose can be seen. Even with the most protective version of the sallet, with eyeslots in the visor or skull, it is possible to wear the sallet pushed back so that the wearer looks out, not through the eyeslots but beneath the lower edge of the sallet. This can provide much better visibility than trying to see though the eyeslots, but the shape of the sallet protects the face from most cuts. In this position the wearer also has some ability to rotate his head from side to side: perhaps 15 degrees in each direction even if the bevor is attached to his breastplate. If the wearer wants more protection he can raise one hand and easily adjust the angle of the sallet so that the sallet overlaps the bevor, while using the eyeslots for vision.
Now compare the sallet and bevor with contemporary alternatives. The grand bascinet offered superior protection, but freedom to rotate was almost nil. Similar visibility to the default sallet position required raising the visor, but this is not as simple as it sounds. If visor pivot is stiff enough for the visor to stay open without being held open, opening it requires considerable force, and probably two hands, and the same to close it. Over time the pivot will tend to loosen, and unless it is carefully maintained the visor will either refuse to stay up without assistance, or slam closed without warning.
Armets had similar visor issues, and the 15th century designs had a weak point between the armet and gorget that was protected only by mail. A plate wrapper could reinforce this point, but only by reducing visibility and freedom to rotate.
Barbutas also had a weak point at the neck, and typically forced the owner to choose a single tradeoff of visibility and vulnerability rather than the multiple aspects offered by sallets, grand bascinets and armets.
Kettle hats could also be worn with bevors. Some had eyeslots in the brim, and these were not very different from contemporary sallets in their strengths and vulnerabilities. Others did not: they offered simplicity at the price of reduced protection.
Jousting helms were the remaining option. They offered superb protection, but the ability of the wearer to see and breath was severely restricted: they were useless on the battlefield.
The 15th century sallet offered an attractive compromise between visibility, mobility and protection, compared to contemporary alternatives.