Here in the U.S., all of them are used (however, "ain't" is used more in specific dialects). They're just results of the evolution of language. New contractions and variations of words are formed, just as all of the others that are considered "normal" (I'd, I'll, I'm, etc) were formed. These reduced forms are usually used more in spoken speech, but it's all the same. It's simply people in different areas and times speaking differently!

In Ireland we use 'gotta', 'wanna' and 'gonna'. You'd never hear people say 'aint'.

With the exception of "ain't," the weak forms are normal for rapid speech. I use them in conversation, though I try not to use them when I'm leveling up my language for a presentation or around professionals. On occasion I've sat and listened to local university students chatting. You would be very surprised at the words that are contracted during rapid-fire dialogue. And they weren't the contractions we teach you in an English class. That's why listening to the language is important.

These forms are from speech. They are not "reduced" forms but lazy way of not enunciating the words correctly. When speaking fast, "want to" sounds like "wanna". The only exception is "ain't". This was used mostly by the uneducated in the past and therefore it is looked down upon etc. For example, "Ain't she sweet" is a name of a oldie song. Like Peachey said, you can live your life without using any of these words. I only used them when I talk disparagingly to someone!

You could live a happy and fulfilled English-speaking existence without using any of them. In general, they're a sign of poor education.