Okay there's a HUGE difference with that logic.
Cassie is notoriously not a good live vocalist. This is a big deal in the case of people who like to pretend that they're enjoying really good vocalists in R&B (See: People Who Think Alicia Keys Even Remotely Carries A Tune.); but in the case of her music, it's not REALLY important, because she's kind of perfectly plasticine. I've stressed this again, but Cassie is in the Aaliyah model of "Robot Turned Singer". Everything is artificial, but that's it's strength. She's just another instrument... She just also happens to possess a really cute body.
However, in the case of Blake, Blake is a SERIOUS ARTIST. He's making REAL MUSIC. Cassie is forgiven for not having enough ability because it's nothing that big. But when 'Measurements' came out for example, everyone was like "HE SINGS TOO!? GOSPEL!?" And I felt like "... Yeah, it's pretty good, If he discovers auto-tune maybe, and tries to take songwriting a bit more seriously." However, he's just as incapable a vocalist as Cassie and he gets blown for warbling "But I don't blame them!" over and over? That's a fucking hook, that's not a goddamn song! Everyone blatantly ignores his lack of ability and pretends it's there, but Cassie being unable to sing was such a fucking travesty!?
And WHY is that? Because of everything Blake HAS done. If Blake didn't have this production legacy behind him, this would've been dismissed as absolute crap, because that's what it is. Blake's 'singer' bits are good in a 'part of the picture' way. Something to occasionally utilize... It's not what Blake does best. But Blake gets automatically praised for just about anything and everything he does so easily that Blake doesn't even know what he's good at.
However, Cassie has not had everything she does loved. Because she lives in a different world... Her music matters less to her fanbase than the fact that A) She may have cheated on Ryan Leslie with Puffy B) OMG SHE CUT HER HAAAAIR! and... then, maybe after another couple of things, her music. When people get to the music, they don't take her as seriously because they think she's untalented (which isn't true), that she's all beauty, that she's only still signed BECAUSE she's 'banging' Puffy. Really, despite the whole 'cult of Cassie', the people who Cassie NEEDS to like her to succeed won't give her any mind.
Actually, that reminds me of how Kelis' is stuck between commercial expectations (Making a Will.I.Am/Guetta produced house monstrosity) and courting 'other' music (The one-offs with Crookers & Basement Jaxx). In the case of Kelis though, she's also a better singer and a songwriter than given credit, unlike Cassie...