I wouldn't make one lotus against rovers to defend mex, since like you said one scorcher can destroy a lotus by itself, so I would make two or more.
But I find lotus very useful like in some situations, since if you mean by spending that metal on more raiders to guard the mex, if you get attacked from a few sides, it would be almost impossible to micro them all well to protect the mex, so making more than 1 lotus to guard mex is the better option and more useful in my opinion.
But if you mean to attack the enemy with it, then there will be less use for lotuses.
Also if you get raided by scorchers and a fencer, lotus is completely usless, because fencer out ranges lotus.
Overall, I think there isn't much use for lotus against rovers unless you have a ton of income and can be fine with losing a lot of them.