Starting next raid event (Sandalphon), BP costs get adjusted to 2 for expert and 4 for ragnarok.
Strictly speaking, Jormungandr isn't the only eidolon with a -10% atk debuff. Yatagarasu should also have the same effect. Yatagarasu's event is 3 months away; so there's about 8 new events plus the 1st reprint before then. So you still want Jormungandr anyway.
I suspect most people will want one of these two eidolons in their team. ATK and DEF down are two of the most useful debuffs, and getting to the -50% cap without the aid of an eidolon is tough without SSR kamihime. Jorm has the added bonus of having a passive buff that's actually useful for its element.
What makes me frustrated most about this event is there are many instances where u are locked out of your own raid. Whenever you try to go back in, the game just kind of crash and ask to reload.
I have yet to experience being locked out of my own raid. I've run a hefty amount already too. Crashing has happen a few time during the raid but nothing about being locked out. Relogging and joining back in without a problem assuming it isn't dead within the next 30 seconds.