"By not mixing T and U traffic on one IF it is likely more overseeable, don't you think?"
To be honest I don't see it as an issue, while I completely agree with KISS and why over complicate things. Not having any settings on a interface tends to confuse new users.. So if they can think of that as network ABC, and then adding vlans on top of that its pretty simple.
But you have a talking point sure. But then your doing it opposite to the end machines - you don't tag the port that end device is connected too. if you do then you have to set the end device to understand the tag.. So no matter how you look at it your going to be running tagged and untagged when you start to vlan. So how is it any different for your router vs your workstation. Just in the router you need to tagg the traffic for the other vlan lans its routing, etc..