I see where you're coming from. I suppose what I originally meant to convey was this:
Production of music is a different level, expecting that from Sibelius is like asking a diner for Wi-Fi, they might be able to, but probably not--- and not like you want it, but, what did you expect, they sell food!
Sibelius does what it is supposed to do VERY well. Sequencers do their job, but there is so much you can do with one, that you often need to "jump through hoops," whilst the refined form of composition that is considered "traditional" is very standardized, the sound you want can be written easily.
Perhaps saying that most sequencers don't function well was a rash overstatement, but my point remains that Sibelius is a pretty solid piece of software, ESPECIALLY after this upgrade! Not only that, but with the addition of ReWire, the more of sequencing is now added to Sibelius.
Quality notation meets quality sequencing--- the music world is reaching a new era of composition.