The link you gave is going to a thread in a forum. It is a question from someone and there are no answers. It is not something on the microsoft pages.
I get the impression that the guy in that forum is just adding printer connections over and over again but not setting default printers.
If I would add printers via login script I would check a few things:
1. check if the printers the person needs are already installed
2. if not installed, just add the printers the person needs
3. check if the right default printer has been set
4. if not set, set the right default printer
The checks will speed up the login script.
Jooel wrote his PriMapState that helps you to know if a printer has been added and if it is the default printer.
Just thinking about a possible implementation:
Code:

Dim $SO
If NOT PrimapState($Printerx) > 0
	$SO = AddPrinterConnection($Printerx)
EndIf
If NOT PrimapState($Printery) > 0
	$SO = AddPrinterConnection($Printery)
EndIf
If NOT PrimapState($Printerz) > 0
	$SO = AddPrinterConnection($Printerz)
EndIf
If NOT PrimapState($Printerx) = 2
	$SO = SetDefaultrinter($Printerx)
EndIf

Hope this can be of any use to you.