I believe the -2 AoE defence modifier is an error and a throwback to when (it appears) they had indirect AoE spells working like single target spells, which is also reflected in the (incorrect) Blast example. Bull suggested a month ago that grenades and indirect AoE are undodgeable and revolve around the 3 hit threshold and scatter, instead.
I think (i) they will errata out that -2 AoE defence modifier completely and (ii) redo the Blast spell example in the magic section referencing the 3 hit threshold for grenade-like-spells and remove the dodge reference.
Definitely something to be clarified in the errata doc, whichever version they want to go with.
I also support the "hit the ground" suggestion another poster had abuot providing some kind of protection against AoE. doubling distance, or giving a flat +5 dice on soak, whatever, just something, anything to give a little bonus against against AoE attacks !