I created this ticket in connection to the http://trac.doctrine-project.org/ticket/479
The resolution was that it's fixed in http://trac.doctrine-project.org/changeset/3183/ - but it seems that it is fixed only in the changelog.
The LEVEL is an oracle keyword and the only way to use the NestedSet with Oracle is enabled quote identifiers.
We used this for about a year or more, but it is not so comfortable and this is the only reason why we are quoting identifiers.
But we started to use Oracle's spatial features and I was surprised that some oracle spatial functions are not able to work on quoted objects, and I started to use synonyms and extra columns with triggers as workaround. But it is so annoying and the patch is so simple