Because you can now get pretty much everything in one gear set without having to macro any gear changes other than your staff.
Caller's Doublet +2: -4 Perp
Caller's Pigaches +2: -3 Perp
Trial Staff +2: -6 Perp
Brings us to -13 Perp, and since other than elemental spirits, avatars don't go higher that 14mp/tick Perp cost, you've capped -Perp with 3 pieces of gear.
Trial Staff +2: -10 BP Delay
Summoner's Bracers: -2 BP Delay
Tiresias' Cape: -3 BP Delay
Add two more pieces of gear and you've also hit the BP Delay cap.
Auto Refresh II: Refresh +2
Caller's Horn +2: Refresh +2
Another piece of gear, and you're at 3mp/tick before things like /rdm refresh. You've still got 9 gear slots to fill up with other stats. Macro in Shareeravadi +2 or Soulscourge for MAB+ where needed, and you're golden. You no longer even have to worry about day/weather for anything other than elemental siphon. And probably have a lot of inventory space back after you store all that macro gear.