It occured to me earlier that the following might serve as a means of allowing dodge and parry to work against hordes without neutralising the horde usually limited number of attacks.
If a character has a reaction remaining, they may make a dodge or parry attempt against a horde attack but with a penalty equal to that horde's current magnitude and so long as there is a possibility of success with this penalty. The penalty is capped at -60 as per the rules on modifiers.
Example 1: Brother Severus with his WS of 45 and chainsword attempts to parry a Magnitude 30 horde of khornate cultists. His parry roll is against 25 (50+10-30 = 25).
Example 2: Brother Severus with his Ag of 50 wants to dodge a Magnitude 120 horde of renegade guardsmen. Dodging their lasgun attacks is impossible (50-120 = -70) so he cannot make the attempt and will have to rely on the God Emperor's mercy.
Pros
It's easy to remember.
There is a noticeable difference in difficulty in evading the storm of attacks from a larger horde. And of course decreasing difficulty as the horde is weakened and dispersed by Kill Team fire. A tangible sense of the horde weakening as it were.
It increases the value of weapons that make parrying easier, since there's more opportunities to parry.
Assault marines can dive into combat with a lot less fear of being torn to pieces by horde melee attacks.
Cons
It weakens the ability of hordes to land a blow at all.
Butchering hordes with counterattack
It still leaves horde attacks as very all-or-nothing. But that is a feature of the combat system as a whole so I wonder if that's a big problem.
Assault marines can dive into combat with a lot less fear of being torn to pieces by horde melee attacks.
Any thoughts?