This would help avoid improper deletion of a Subtask when actually a DETACH is appropriate. Currently you have to be within the Subtask to see the multiple assignments - and even this is hard to decipher the multiple assignments at the top of the page. This is all even *if* someone is being sensitive to the fact the Subtask might belong to several other Parent tasks. Thus you have to "open" every task and make sure it is not assigned to other tasks before deletion.
Alternatively, when choosing delete, a warning could pop-up cautioning that the Task is assigned to multiple other tasks and that a Detach may be more appropriate.