Automation Interface Objects

The snippet of code below is from a program that I now maintain because
programmer has left the company. The program was developed in AutoCAD 2002
and has been 100% operational until AutoCAD 2005. After spending numerous
hours debugging, I discovered that the statement: "Set oPViewPort = vItem"
was causing the program to fail. The error message was "Object variable not
set..."

After discovering where the program was failing, I used the
(vlax-dump-object) to snoop around. This is when I discovered the
IAcadPViewport2 Interface Object name. I ended up getting the program to
work by simply changing the IAcadPViewport Interface Object name to
IAcadPViewport2

After doing further investigation, I ended up changing the statement: If
TypeName(vItem) = "IAcadPViewport" Then...TO: If TypeOf vItem =
"AcadPViewport" Then... I was told that this would be a much better
technique.

Here's my question. When are AutoCAD Interface Objects versus AutoCAD
Objects used? For Example, (IAcadBlockReference versus AcadBlockReference),
(IAcadPViewport versus AcadPViewport), etc

Here's an additional comment. I think the lesson that I have learned is that
from a maintenance stand-point, it's not good practice to use IAcad...
Interface Objects. Please comment.


Original Code:
Dim oPViewPort As AcadPViewport
Dim vItem As Variant
ThisDrawing.ActiveSpace = acPaperSpace ' switch to paperspace
For Each vItem In ThisDrawing.PaperSpace ' get viewport object
If TypeName(vItem) = "IAcadPViewport" Then
If UCase(vItem.Layer) = "NOPLOT" Then ' use layer name to identify
Set oPViewPort = vItem
End If
End If
Next vItem