[derefr]

Because you can be one without the other, and people frequently assume that because you're one, it means that you're the other--and predict your actions based on that (for example, by buying you stupid geek toys when you are merely a nerd.)

[arrrg]

So, why not eschew those categories altogether and reject them outright?

It seems to me they are too, well, mushy to be really useful.

[derefr]

I don't understand what you're saying. The reason it is useful to make this distinction, is to explain to people that don't see the distinction (and thus assume that, for example, someone who likes math watches Firefly; or that someone who collects MTG cards will be able to fix their computer), that in fact there is a practical difference between the two categories, and that they should keep them in mind so as to not offend people that aren't both by assuming they have traits of a group they don't.