Why get a laptop when you really want a desktop?
I imagine, it's because if you haven't done something previously you won't know if you'll like it or not.
Like the OP, on one hand, I think the flexibility of a notebook computer is pretty neat, but on the other hand, I like having a decent set of speakers, an external hard drive, etc. Since I've never had a notebook computer before, I hadn't really given much thought to the OP's situation presented above, now that it's been brought up, I too wonder if it might be a detriment to getting a notebook computer.