Cons....none that I can come up with in real world fishing scenarios. Ya I suppose for true, like getting into the competition disgrace casting world type scenarios then a line that is a bit too heavy can be detrimental, but for 98% of us I don't see any negatives.
Pros: Easily loading up a rod quickly tends to help most of us cast a bit better, especially on the sound where we are doing more distance blind casting than about any other fishery I can think of. A heavier line, IME, often leads to fewer false casts for us average casters. Less false casts over the course of a full day adds up to a lot more fly in the water time, which over any other single factor is the biggest contributor to catching PS fish on the fly.
Being able to quickly load and shoot a 40-50' cast or less when that fish jumps nearby, or that pod of salmon pops up and is crashing bait for a few seconds is also extremely advantageous. Over lining does nothing but help those scenarios, and in a full day of fishing the sound, especially from a boat, can lead to a handful of hookups that may not have occurred if I couldn't instantly load up my rod and get a cast off.