If it wasn't for the effect, I wouldn't use the spell. It would be more fun for me just to kill conventionally with attacks and warps. But I'll definitely want to use it and have partially "decayed" mobs running around . Don't really care that it doesn't look like real decay, or if it's weird on mechanical enemies or anything like that. It would be cool if more realistic, but I guess I'm not that picky.
I figure it's like that for technical reasons more than anything else. If it, like stripped away layer by layer or something to that effect, it'd require the creation and loading of new textures which there really isn't room to do. Personally, I think the effect looks pretty cool/creepy. Sometimes, you get some weird artifacts (like on mechanical stuff) but, whatever. Don't know that the enemy doesn't re-inflate if you leave it alone for a few seconds, either.
But, anyway, as for use, I think it'd going to end up being an end-game only trash clearer for the most part. Everything in that video is significantly lower level than Noctis. I could totally see using it as I'm just trying to get through some trivial content or for farming and whatnot.