Let me start with the reason most players will start using a Summer Pirate - Treasure Hunter; Booty Boost; and most importantly, Golden Eyes. These three skills each increase the chance of looting an item or weapon from a battle, starting with a 50% boost for Treasure Hunter, and increasing to a 200% boost with Golden Eyes at level 100 (with 100 Summer levels).
The loot bonuses aren't the only reason to use a Summer Pirate however. With good attack and luck, he can be a decent damage dealer. He is able to use powerful single targeting weapons like Machetes and Swords and can also use both splash-type weapons - Grenades and Blunderbusses.
In the skills department, the Summer Pirate still has places it shines. Posh Bliss casts Euphoria on all team-mates which can be very useful against a boss or tough battle. Nugget Shower deals decent damage to all enemies which can help speed up a run. Bribe and Bait are both interesting - they have at least a 50% chance to charm an enemy, but unfortunately at low levels they wear off very quickly. Power Monkey gives a useful haste, and has a chance to boost attack by a noticeable amount. If Power Monkey fails to boost attack, the player can use Grog Shot for a smaller attack gain, and also some defence and accuracy boosts.
Now I'll come to the weaknesses of the Summer Pirate. Most noticeable in an endurance context (Arenas and Endurance Isles) is the lack of a heal, something Summer is usually well known for. He also has a low accuracy, so you might find yourself missing rather more than you'd like - though this can be offset somewhat with Grog Shot. With moderate defence and weak resistance, he also takes quite a bit of damage, though this is offset a bit by the reasonable hp.
Overall, I think the Summer Pirate is a great class that will always be in high demand. While it does have its weaknesses, these are rather outweighed by its strengths. I give the Summer Pirate a solid 8/10.