Everyone should own a few bitcoins.
my rational:
the first bitcoin puts you in a group with a maximum membership of 21 million. The second coin splits this group in half.
I think most people that aren't living in abject poverty could afford to lose $300. The time is near when very few people will be able to "afford" 2 entire bitcoins (I'm thinking of people I know who have never had $3000 all at once in their whole lives).
Personally, I recommend that everyone own at least 2 as soon as possible.