One reason to choose Nasty Plot over 3 attacks is because the offensive coverage between Escavalier and Slowking is already great and not much need for Fire Blast (unless we really wanna hit Ferroseed THAT badly). We also don't know what we're up against and the extra power provided by Nasty Plot is a safe way to fall back. Nasty Plot also turns many 2HKOs to OHKOs which is great. We also want to be dealing enormous chunks to anything that switches in, resist or not and +2 SpA will take care of that. And while NP is hard to set up, if we do get the chance it's gonna rule and I don't see how Fire Blast helps us other than covering the Grass-types that Escavalier can already punch through.