[GMS.VL] just a mercedes i know, doing what mercedes do.

I don't understand the part about "If you did this prior to the Legends change, then it doesn't apply to you." as it was CHEAPER to wash like this (for the same results, that is).

Yes.. That's the point. If you washed before you're wasting less money and it was actually more beneficial to do it because it wasn't as difficult or as bothersome to do it, hence desperate.

would you say the person in the video needs more cubing? or wouldn't be able to cube if they wanted to?

What does this have to do with anything? People rarely HP wash in kMS as it is.

For example, at empress, I can now use an exception hero + dex boost potion as opposed to using a health boost.
I can also use spikes royale on PB + his minions when he's in reflect.
I also lived in an empress run that I failed earlier today with Villain, I was able to get empress to summon his 5 instructors before I ended up DCing from FJ.

Basically:
Is it useful? Yes. Is it required? No.

1. You wasted extra amount of money for a potion that gives you 70 dex? In comparison of using a potion that would save you all that money?
2. Why would you use Spikes Royale at PB, what purpose does this have?
3. As seen on the video with less HP than you, surviving Empress isn't really an accomplishment.