I disagree with this sentiment, but I also think it comes down to how you approach music.
For me at least, music originates from my imagination. It starts in my head, not from playing an instrument, so to speak. I say this as a composer, not as a musician.
Sure, if you really mastered an instrument i.e. Push 3, then you could use it as a form of expression. But when you're composing, often you're trying to tame many different instruments and ideas at once, so I find it much easier to do it all in my head first, and then think of the quickest way to translate it all into the computer, and for me that's the mouse and keyboard.
I'm also primarily a song writer/producer, so for me I think in terms of composition after the fact i.e. once the lyrics have been written. But yeah, I certainly do appreciate those who can utilise platforms like the Push 3. However I would still argue that even if it is what works for you, it's still ultimately slower. Again, nothing wrong with that if it works for you.