Open in another app is a good feature, but I'm not a huge fan of using it in place of native selection and transformation tools. For one, running two applications takes precious resources from my non-expandable computer. (Stupid Cintiq.. apparently you can add more memory, but you will void the warranty and scratch it) Two, you have to do the edit out of context of the entire image.
Setting up a group, making a copy of your layer, moving it into the group, creating a new layer, setting it to destination out, painting in what you want to keep, merging everything... it's a rather roundabout way of doing things, especially if you like to shuffle things about a lot.
What I'm trying to do here is to send a message to the developers that this user at least, would be grateful for a very simple select and transform solution.
Don't worry about free-form selection, magic wand, or keeping brushes within the selection. A rudimentary rectangular selection will go very far.