I believe they used Weakness because its heavy penalty on HP/MP while it also gives 100% Slow, so an enemy can use it and then stack its normal debuffs on top of those, allowing for a ton of Slow and a ton of HP/MP Down. I cant say for sure that it is another ailment, its probably the exact same, but I would hope they could allow it to work with different rules depending on how its causes, kinda like Reraise. Some forms of Reraise wear off due to certain events like entering a Battlefield, while others do not, I assume they are both the same thing, but their causes change their abilities to be removed & such. If the same kind of thing could be done with Weakness, it should be able to be removed with a spell, but if not, then SE simply needs to be smarter with their implementation of ailments.