I don't see anywhere in the encounter design section of the DMG anything even faintly resembling an encounter template made up of nothing but minions. It isn't a type of encounter that is advertised as being balanceable by the encounter design system.
This is true, and a good point.
The DMG does say:
"Sometimes you want monsters to come in droves and go down just as fast. A fight against thirty orcs is a grand cinematic battle. The players get to enjoy carving through the mob like a knife through butter, feeling confident and powerful."
But it also says:
"Minions are designed to serve as shock troops and cannon fodder for other monsters (standard, elite, or solo)."
So the DMG certainly doesn't recommend using encounters made up of just minions.
In your examples encounter 1 and 3 are throw-aways that exist for dramatic effect. They could be challenging or not of course depending on how big the "horde" is, but the XP system isn't going to balance them well. I wouldn't count on those encounters doing much and if I wanted them to have an impact I'd probably make them initial waves followed immediately by encounter 2 and 4.
Again you make a fine point.
The encounters are in a sense "throw aways", but I would argue that "dramatic effect" is a very important aspect of encounter design and overall encounter (or adventure) flow. Yes the horde encounters would be transitions between the main (and more meaty) encounters but they might still cause the PCs to expend resources if handled correctly, and would certainly add to atmosphere.
The MMII upgrade to minion damage, coupled with using more minions keeps minion encounters threatening and exciting. It also allows the players to feel like heroes by facing hordes of enemies swathing through them.
I do accept your point that RAW the encounters wouldn't work anywhere near as well though as I houserule the minions xp value and build my own minions. So your point stands as true.