I haven't played the JMP event, and that's what I learnt

Asylamb

Correct me if I'm wrong, but that's how I see the event structure:

1. When you play it, you select a PW-subdeck and a Theme-subdeck. The Theme-subdeck can be of any color and/or strategy and might have good synergy with the chosen PW, or none whatsoever.

2. After you play (or win?) a single battle, you are bound to get the new cards from the selected PW- and Theme-decks, after the event ends(?). All PWs offer at least the new Thriving Land, but many players might also miss the Vanguard cards from those decks. So far, the only Theme-deck to lack a JMP card seems to be "Spirits".

3. I guess that if you have chosen "Spirits" subdeck this time, you got new cards from another random subdeck. And if you choose the same decks next time (for example, just because they worked well for you), you'll get random subdecks for your end-of-event reward.

4. Considering that there are 20(?) subdecks total and you use 2 of them per an event run, it will take 10 participations (3200 crystals total) to get all JMP cards.

5. If you still choose to play the event after that, you should be getting the dupes of the manually selected subdecks. In that case, the most optimal choices are (the number after the deck name is the orbs quantity you should get if you already own every card in the deck):
* Garruk-1160 for the PW-deck (2MR + 1R, and arguably the best abilities)
* One of the following Theme-decks for another MR dupe
  * Unicorns-670/Mill-620/Phyrexian-580 (JMP dupes, I don't know if the "newest set" orb rule applies to them)
  * Spirits-670/Angels-580 (regular Legacy MR dupes).

So, potentially, you can get up to 1930 orbs, which is useless for veterans and/or VIP members, but is still valuable for F2Ps.

I haven't looked up the progression rewards yet, but I guess those are not enough to encourage the players.

MetaBunny

Not quite, unfortunately. Crucially, after selecting your PW, you're offered a choice of 3 random themes out of the 15. (For example, I choose Garruk and was offered Pirates, Spirits, or Seismic.) So it could be several runs before you're even offered one of the three themes with a mythic.

Mainloop25

The progression rewards are slightly better than NOP, which is to say they are terrible for the price you pay