Pros and Cons of turning off Smart Definitions

I am trying to decide whether to turn of Smart Definitions.

 

Pros: my understanding it is expands signatures which I assume increases known virus detection. Cons: ?? What are the

 

Is that correct? Other Pros?

 

 

Cons:?

 

I know the initial download is larger - are future downloads larger too? Scans take longer? Will hard drive spaced used increase dramatically?