I think the best example of how step-ups give no guarantee is Lustjaw (was him, wasn't it?) spending $10,000 on them without getting Lunarie.
If you spend that kind of money on a dream chest you WILL get all the cards in the chest, I think emptying a dream chest costs about $4,000-$5,000 iirc? And you will likely get all relevant cards before that.
But, yeah, Amasuro said it, buy packs for items to rank, and maybe to get an event unit. Gambling with no goal will not get you much.