The differance between buying DS vs buying Orbs is the fact that orbs aren't used to get new units, which is the basis for this game.

You play the game -> You use up story stones -> You want to keep summoning but you don't want to wait ages for stones to do summons -> You buy stones

Meanwhile, you use orbs to buff up your units, which helps out people who are f2p and have units that aren't really the meta in terms of LS. Now the thing is at some point, you will run out of free stones from events but. Yes you can get stones from special login bonuses but due to how the game works you're never guaranteed something good, with orbs you can bet your ass that what ever unit you use orbs on will become stronger.

You will never run out of orbs. You will ALWAYS be able to get orbs, be it from the orb event, WT, Battlefield shop (if that ever hits glb) and orb missions that are apart of celebrations.

Now if they didn't make it so you're paying $20 for 1 run of the orb packs that would be great. At the very least, make it so you can unlock one 1 non-dupe tree. when you think about it $20 doesn't seem like a lot, but combine that with the shit amount of orbs you get and, the games horrible currency conversion for other regions it is a pretty bad deal.