I think this is a valid point. For non-performance-critical code where readability is more important (eg analytics) the additional language features could be nice.

In my experience, 9/10 production use cases are covered using a path as primary key with prefix matching (select path,data from tree where path='a/b' or path like 'a/b/%') - as a side effect your subtree nodes will often be on the same table page (mySQL by default, pgSQL when using CLUSTER).