Nebulous
Legend
I wonder if the biggest reason behind the wording in the DMG is to give a bit of a boon to spellcasters when they cast a spell? By making your Readied action occur after the triggering Action, you can no longer shoot a caster as he casts a spell, potentially ruining it. This is important to know, as there could be times a DM (one playing particularly savvy foes), might have five archers Ready actions to shoot the spellcaster if he casts a spell, which would force five separate Concentration checks. Now, the caster would complete the spell, then five arrows would come his way. Equally scary, but perhaps less frustrating?
That's my guess, to make it harder to disrupt spellcasters. I'm really glad I read this thread, it's been helpful.