Pros: It depends
Cons: It depends
Pairing is such a personal thing to be prescriptive about, it totally depends on the people involved.. An engaged pair can create some of the best code for your product, work fast and be less prone to error, on the other side though, the wrong pairing creates disengagement, lack of ownership and meetings with HR (no-one likes those).
In my teams I have found it best to encourage pairing where appropriate and let it be natural