If you query xdebug with "feature_get -n breakpoint_set" you will get a reply:
<response command="feature_get" transaction_id="2"
feature_name="breakpoint_set" supported="1"></response>

Shouldn't the text data of the tag be set as well?

ie, the default case of DBGP_FUNC(feature_get) should be changed to:
XDEBUG_STR_CASE_DEFAULT
xdebug_xml_add_text(retval, xdstrdup(lookup_cmd(CMD_OPTION('n')) ? "1" : "0"));
xdebug_xml_add_attribute(retval, "supported", lookup_cmd(CMD_OPTION('n')) ? "1" : "0");
XDEBUG_STR_CASE_DEFAULT_END

?
XDEBUG_STR_CASE_DEFAULT
xdebug_xml_add_attribute(*retval, "supported", lookup_cmd(CMD_OPTION('n')) ? "1" : "0");
XDEBUG_STR_CASE_DEFAULT_END