From the user's point of view, the placement of the dropped item is dependent on the task the user is doing and the application or context the task is in.
In File Manager, if the default is set to As Placed, then icons are placed where they are dropped. If the default is set to Sorted Grid, then a dropped icon is automatically sorted and then placed, which means it may not be placed where the user drops it.
In some cases, where the dropped item gets placed is not a criteria. For example, Front Panel controls require only that the dragged item be over the control to activate the drop zone.
In the Compose window in Mailer, the placement depends on what is being dropped. If the user is dragging a piece of text, then the text is inserted at the drop point. From user testing, this is what users expect. If the user drops an icon, a file, or a buffer, then the contents are included at the insertion point. This mirrors the behavior the user gets when the user selects a file from the Include File Selection Box.
You should determine appropriate behavior for your application based on what type of tasks the user is doing.