The short answer to the question of whether or not to add people that you don't know to your Facebook is: it depends.
I'll give you 3 reasons why you should add people that you don't know, and then I'll give you 3 reasons for why you should not add people that you don't know in person.
3 reasons to add someone you don't know:

1. It could be a new or potential client or person that you might do business with at some point in the future.
   
2. It could be someone that you do know, but they've changed their name due to marriage/ divorce, a religion change or a sex change operation.
   
3. It could just be a friend of a friend that you've met in person already, but don't recall them or someone who's Facebook name is different than their real life name.
   I have several people that I have met in real person, but their Facebook name is something completely different than their real life name.
   

3 reasons to NOT add someone you don't know:

1. It could be a criminal of some kind.
   Someone that is looking or waiting for you to update that you've left your house alone, or that you're kids are alone at the house or someone that looking to commit identity fraud.
   If you don't know them personally, how can you know their intentions?
2. It could be a potential client, or employer's representative looking to snoop around in your account, looking for reasons to not hire you.
   I'm very politically opinionated towards a particular view, but people that know me in person are never offended by anything I say or post.
   Someone that doesn't know me or understand my sense of humor may be very offended by some of the things that I say on Facebook.
   If they are offended, how likely are they to hire you?
3. It could be that ex-girlfriend, or boyfriend, that you never wanted to speak to again that has somehow found you.
   

It's up to you to decide which options speaks better to you.
Personally, I do not like to add people that I don't know, or that I have not met in real life to my Facebook account.
I feel safer knowing that I have met, or know, everyone of my friends on my list.
Of course, your mileage may vary with that approach.
There is a third option, which is to create a Facebook fan page that you can advertise to people.
A page, allows you to separate your personal and private profiles from each other.
Also, while your personal profile is limited to 5000 friends, the fan page is not so limited.