It could have gone the other way. When we modelled this in the past, the clinical people were pretty adamant that they wanted any text item to be seamlessly replaceable by a coded item. So we did it that way. More recent discussions (e.g. this one with 72 replies 
came to the conclusion that if we had our time again, we would model it like this:
- DV_TEXT (abstract)
- DV_PLAIN_TEXT
- DV_CODED_TEXT
So if you wanted to have a value in an archetype with a constraint meaning ‘text item, coded, or plain’ (aka coded with exceptions or CWE in HL7v3, v2), then you would just constrain the value to DV_TEXT. Otherwise you constrain it to only coded or only plain text.
We might still do this…