I tried posting the following variation on the example from the BBCode help page:
Regular text [INDENT]Indented text [INDENT]More indented[/INDENT] Less indented[/INDENT]
Testing a more complex version: Regular text [INDENT=1]Indented text[/INDENT] [INDENT=2]More indented[/INDENT] Less indented
I presume posting the version displayed by the plain text editor will cause the fourth line to become unindented, necessitating further editing of the post to get it right.
If we really want people using nested INDENT tags, then perhaps the BBCode parser should be fixed so it correctly decrements the depth parameter and uses that value to indent text appearing between nested /INDENT tags. I don't know if this is possible, though.
An easier solution would be to avoid promoting the use of nested INDENT tags, as I suggest here.