schematron and conformance

When I define my own Schematron constraints and add them into an ODD, 
their effect is always to reduce  the set of documents which will be 
considered valid, by comparison with the set that might be considered 
valid by TEI All, or by a  version of my ODD without those constraints. 
Hence I infer that schematron constraints are always going to be 
restrictions rather than extensions of an existing schema, which means 
(I think) that adding them has no effect on the TEI Conformance of my 
ODD or the documents it validates. Good.

But what about the constraints which the TEI itself defines ? If a 
document is valid against TEI All but fails some TEI-defined schematron 
constraint is it no longer TEI conformant? The current definition (in 
chap 23 of the Glines) says nothing on the topic. You could argue that 
the object of most (or all?)  TEI-defined schematron constraints is to 
test some semantic constraint otherwise expressed only loosely in the 
prose, and conformance with the TEI semantic model is also a requirement 
for conformance, so a document which fails the schematron test is ipso 
facto non conformant. You could argue that validation with schematron is 
an optional additional extra which shouldn't be required of all TEI 
users, since not all validating software supports it.

Just wondering if there are any strong views out there ...