Hi Eduardo,
honestly, I don't even know how to answer that question, but I'll try:
It's like asking "Why should I bother learning German if I already know English?" Maybe you want to talk to people in Germany, Austria and Switzerland? Maybe you want to order a meal at a restaurant there? Maybe you want to move there or open a business?
Just as learning new spoken languages is either useful or necessary, depending on what you want to or have to do, so is learning new programming languages.
You want to use Ruby on Rails? Well, you'll need to learn Ruby because Rails happens to be written in Ruby.
You want to use SproutCore or jQuery? Well, you'll need to learn JavaScript.
You want to develop for the Mac or iOS? Well, you'll need to learn Objective-C because all of the Cocoa frameworks are written in Obj-C (or C).
So, to sum it up: You don't have to learn Obj-C if you don't want to develop for iOS. If you want to stick with Java, you should check out Android. But if you do want to develop for iOS, you have to learn Obj-C (unless you use something like Titanium or make HTML5 apps, but then you'll have to learn JavaScript
).
Cheers,
Johannes