For every mountpoint Ubuntu creates
.Trash directory when a file is deleted.
1000 is the UID of the user that created the bin, so every user can have their own.
Is it necessary to keep this folder ?
I think it is your choice. It just a bin folder, If you need a bin folder for your mount HD you can keep it.
If you want Ubuntu prevent creating this directory:
Don't use the delete button only (Otherwise the
.Trash-1000 folder will be created)
Press the key combination Shift+Delete together to delete, then Ubuntu won't create a
Note for 12.04: Under 12.04 i noticed that
.trash directory recreated even if you deleted with Shift+Delete
Note: If you delete this directory files and folders this way they are gone forever