I'm currently working on an iPad/tablet app. I want to decide to use or not to use drag and drop in my app. As I'm looking at iPad's native apps there is no drag-and-drop functionality. Is it a bad UX to have drag and drop in touch interfaces?
Update:
This is my app layout. Basically user will be able categorize users by dragging and dropping them into different categories.
If you're confident in the quality of the touch-screen, your design is a good one.
Some points to note:
In general, I'd advise changing the design to one where the items start in the middle and move (a smaller distance) outwards:
download bmml source – Wireframes created with Balsamiq Mockups
Your primary consideration should be whether the UI behaves the way the user expects. If it's intuitive, drag and drop away. You can use hallway testing to check if your UI is intuitive. In that situation, dragging and dropping appears to be the most sensible as well as the easiest course of action, so there are few (if any) reasons you shouldn't use drag and drop.
The only other thing you might want to consider is strain caused by too much movement. If the user has to drag something very far (or very often), it's a different experience than using a mouse. With your updated question, it seems like this shouldn't be an issue. One way to counteract this, however, would be to allow users to select groups of people that can then be dragged and dropped.
Finally, when you say there's no drag-and-drop functionality in the iPad's native apps, you're missing two things. First, there probably isn't a need for it (so if there's a better way to implement your functionality, do that instead). Second, iOS does use drag and drop. When you're rearranging icons, placing them in smart folders, etc., you're dragging and dropping.
updated to reflect changes to question
It's bad UX to not have drag and drop in an iPad app. Tablets are all about direct manipulation. The D&D is so native to the apps that you don't even notice it when you say that there's no D&D in iPad apps - basically most of your interaction with the device is pure D&D. For most of the actions that you'll provide, users will expect there to be a direct manipulation - i.e. drag and drop - way of doing it (as long as it makes sense, so no dragging letters from an alphabet list to replace typing :).
Mobile apps are essentially all tough/drag and drop interface. The concern with your layout is whether it will be too much work for users to drag and sort all those items compare to tap highlight and bulk move with a button.
Judging from the screenshot you provided, it's probably more ideal to not ask the users to d&d sort that many items as appeared.
One thing to keep in mind is that touch-move-finger is also used to scroll and page swipe. So if you don't need scrolling you can use touch-move-finger for dragging objects. But you can't have both (comfortably).
