Holding the fiddle on your chest:
Disadvantages
1. Your arm is straighter, and your fingering hand's further away, so you may have to have the fiddle neck lying on the heel of your hand instead of away from it as it's "supposed" to be. Many of the great Old Time players hold it just like this with no apparent disadvantage to their playing.
2. Your ear is further away from the f holes, so hearing your own fiddle is more difficult in a noisy session.
3. If your arms are short, it's very difficult.

Advantages
1. It's a lot easier to sing and hold a conversation when the fiddle's on your chest instead of jammed under your chin.
2. Many people find it more comfortable.
3. It looks cool, if that bothers you.
Cheers
Ray