Sloane's problem
I mentioned before that this movie might be all about 'problematic people' - half of them are mentally ill or psychologically traumatized or emotionally disturbed, or whatnot. And Ferris is a sociopath.
I forgot Sloane's problem, because of Sloane's problem.
You see, Sloane's problem is that she has no personality.
You could've replaced the actor with Claudia Wells, and nothing would've been lost. Both give similar performances (Claudia Wells as Jennifer in Back to the Future - both are good at staring into the distance and looking at the hero of the story).
But in actual fact, the actor could've been replaced with any 'eye candy' - let's face it, that's her only reason and role in this movie. It would've been much better without her - she makes Cameron seem like a third feel, whereas the whole 'day off' is actually almost done because of Cameron.
My point is, if you have an eyecandy girlfriend that you plan to spend your 'day off' with, why take 'excess baggage', like Cameron, to spoil the 'GF fun'?
And if you plan to spend it with Cameron, why bring your eyecandy girlfriend to make Cameron feel like a third wheel? This movie could've worked better as a 'buddy movie', but I guess there's some kind of dynamic in this weird social triangle. Also, she can be useful sometimes (speaking to the Maitre D' on the phone, for example, and flirting with Ferris's father, making him forget that he might've seen someone resembling his son).
If you look at things from her perspective, would she really enjoy having a negative whiner like Cameron in their group, or would she rather be spend the day being intimate with her cool and high-status sociopath boyfriend? I mean, come on - there's no way she cares about museums or baseball (though I can see her enjoying the fancy restaurant), or all the 'good, wholesome fun' they're forced to do because of Cameron.
A young teenage girl like that would most likely rather just party with her boyfriend than waste her rare day off being an unpaid therapist to a depressing hypochondriac.
Too bad Ferris couldn't help her in the end, because building a whole personality from scratch would be too much for even Ferris.