If by motivated you mean "want to spam your site specifically at any cost", you're right.
But running javascript multiplies their processing costs substantially and it also means that at that point their costs can be driven up far higher simply by making the computation required to post higher - it doesn't take much - say a few hundred milliseconds of hash calculation on posting - to suddenly tie up a lot of resources for someone trying to spam as many people as they can for as few resources as possible.
For any spammer that has softer targets it makes little to no sense to bother.
But running javascript multiplies their processing costs substantially and it also means that at that point their costs can be driven up far higher simply by making the computation required to post higher - it doesn't take much - say a few hundred milliseconds of hash calculation on posting - to suddenly tie up a lot of resources for someone trying to spam as many people as they can for as few resources as possible.
For any spammer that has softer targets it makes little to no sense to bother.