The BNS freebies are a great idea, I certainly recommend checking those out. I have BNS in PDF and Print, and one of the really nice things about it is that it is strictly a toolkit. As such, it contains several tools to enhance your Sci-Fi game, or create your own homebrew. As a Sci-Fi toolkit, the bulk of the classes are obviously for use on some sort of starship... whether a military cruiser, or a single manned armed craft, or a tug...
On the other hand, as to the claim that all the classes are starship occupations, I would point to a few that Psion listed.
Doctor, Marine, Mercenary, Scientist.
Of the nine listed classes, 4 of them are quite useful in situations other than on a starship. Nearly half of them.
This certainly isn't to say that they are NOT useful on a starship, but the flexibility to use these classes in places other than a starship is a fact. In the end, it is (of course) your decision. You might even decide to go with both (I'm using Blood and Space to supplement Star Wars, as well as getting ready to create a homebrew campaign based on BNS).
Some things about Blood and Space, even taken by themselves without the rest of the book, are worth the price of it:
The feats. Well written, and incredibly useful.
The Space Combat rules. The first ones I've found that are actually usable.
The Classes. Starship or ground bound, or a mix... take your pick...
Starship Construction rules. What can I say? They quite simply rock...
Now... I'm not going to bash T20. I'm not even going to say BNS is better than T20. I don't HAVE T20, and thus I am not able to make that determination, in good conscience (though I won't speak for any other posters). I honestly see no reason to get it, though. BNS fulfills my D20 Sci-Fi wants.
I hope these posts have been useful to you.