Extended file attributes can be used to implement a large number of useful things, many of them enhancing security, e.g. access-control lists. However, IIRC OpenBSD has chosen to also not implement ACLs.
In any case regardless how useful or not useful extended file attributes are considered to be, deciding to not implement them in the main file system used by an operating system has the immediate consequence of disqualifying this operating system for the use on a file server (a.k.a. NAS), which is an application domain where the *BSD operating systems have been traditionally very good.
The reason is that whenever such a NAS would have Windows, Linux or FreeBSD clients, transiting any file through that NAS would potentially lose data.
In general, in the documentation of any file system, the lack of support for features like extended attributes or access-control lists must be displayed very prominently, to warn any potential users about the risk of data loss during copy operations (because the file copy commands are usually stupid enough to not inform the users whenever they are stripping file metadata, so such a loss may be discovered only when it is too late).