I think the point is that when there's a drawback it has to be worth it. The levin sword is good, but arcfire is slow and most of the time it ends up biting me in the butt when i use it. Need to be really good at reading. And people can even dodge out of it, and all that.
It's not always easy to balance a game. But i think with more experience they should be better at it now.. hopefully
I'll admit that Arcfire is a bit tricky to pull off sometimes due to how slow it is, but it still is worth it when you land it because of the amount of follow-ups you could make because of it, like F-Air, U-Air, B-Air, U-Smash and Thoron—They're all very strong kill options.
Personally speaking, the durability drawback is worth it because of the amount of uses his spells have. Most projectiles are either meant as disruptions or kill options, but Robin's Thunder spells alone cover that.
Thunder is a quick, weak move that can be used to interrupt the opponent to either take a breather or continue a combo.
Elthunder is a general projectile that has decent range and killing power, which is useful for sniping opponents when they're off-stage.
Arcthunder is good for baiting out shields for a grab or Nosferatu or even a potential shield break. In Smash 4, it was a bit better—it was much easier to get grabs, Nosferatus, footstools, Arcfires, and even Smash attacks off of it.
Thoron is a strong kill option that covers the entire horizontal area and can be used to punish tech rolls, and can even be used after Arcfire.
Arcfire is a projectile that helps maintain neutral and setup for kill options. If it misses, it can be used as a temporary shield from some projectiles. It also covers ledge recovery options pretty well. Arcfire was arguably better and easier to pull off in Smash 4 as well—footstools, Arcfire chains, etc.
Elwind as a recovery option has a lot variety, it can be used to mainly get horizontal distance, vertical distance, or a mix of both. It even serves as a great spike move, since you recover while doing it. Its durability is easily offset by the fact that it is a great, if slow and predictable, recovery move, and that it recharges in 5 seconds anyway.
Nosferatu does a decent amount of damage and recovers a fair bit of health. As far as command grabs go, it's pretty good. It's great against shields and rolls, and it's a pretty decent ledge mixup option since most people would be caught offguard.
Of course, being interrupted mid-cast does still take away durability points, which is annoying sometimes, especially against Megaman and Villager, and Robin's projectiles DOES clank with a lot of the other, weaker projectiles when it really shouldn't, but it's not too bad. It's a minor nuisance at best.
TL;DR, Robin's spells have a lot of different uses in them, and I think that alone offsets the durability mechanic.