Twitter only gives you 140 characters to type out your message (a brief history of character-based communications here).
Using 140 characters or less takes a little bit of practice to effectively communicate your message w/out losing any of it’s vitality. It’s true that there are some services that are built into some Twitter clients (like UberTwitter) that can extend your message over 140, and provide a link to the rest of the message, but you may lose your audience, especially if pertinent parts of the message come after the jump.
Another reason not to use the full 140 characters is because your tweet will be exponentially less attractive for a Re-tweet.
Read the entire article by Louis Pagan at Latino Rebranded.