Well, this is a coding tip and I beliеve its a nice and useful tip for all max scripters. It's about some limitation of addModifier function that does not preserve the sub-object selection that might be active in the modifier stack when adding the modifier.
Btw, it's a bit strange to see that this limitation of addModifier lie in Max from version 3 to the nowadays, nm, however, if we want to preserve the sub-object selection when we add a modifier, the help docs suggest to use the modPanel.addModToSelection() method. That's correct, but for non scripters need to say that we try to avoid modPanel interface in our coding always then its possible, mostly for good performance but and by other issues.
So, the solution that I have (and share it into Autodesk Area forum) to preserve the sub-object selection (without modPanel) is just to add Turn To Mesh modifier. That's it. Simple and effective.