Pros:

Looks awesome when the Smoker dies (Cloud of yellow bio hazard smoke) 
Makes the Smoker stand out more from other infected.
Adds more of a challenge as when the Smoker dies it adds a thick cloud of smoke which is hard to see through in tight spaces

Cons:

None

Conclusion:

A really good mod for the Smoker, I've had it installed for months and have not had any problems.