Q: Why should add a link to joy.ly to my tweet?
A: Not all of your followers can see your joy.ly tweets correctly (more about this in next question). But anyway they'll see link to your tweet on joy.ly click it and go to joy.ly to see your tweet correctly.
Q: Will all my followers be able to see all these symbols in my joy.ly tweets?
A:
Not everyone. It happens because not all OSes support Unicode fonts properly.
We know for sure that users who use the following tools will see joy.ly tweets correctly: