I can't give you 5 reasons that haven't already been listed, but I personally would go basic before completely deleting my account. I say this because I have developed friendships within the game and enjoy playing and so on with these people, the kind that just aren't as fun as simply being a friend of theirs on a social network or what have you.
As far as what TSC says about what you're looking for in a game, I heartily agree. It's all in personal preference. My boyfriend plays Warcraft and LOVES it, I see how it's fun and usually enjoy watching him play, but have no desire to create an account of my own. He is the same about POTCO. Sven says it nicely...it's all in your own reasons for staying or going.
I have some faith that with the 4th movie coming in the future they will keep the game going for a while longer and at the very least try to attract new players with new content. So maybe I'm biased to this game. /shrug. Hate to see a good pirate go but it's very understandable.
