There are three packaging tools supported. deb, ipkg and rpm. You are free to use one or more of them for your own projects. Bugs will be fixed for them as well.
Why does Yocto Project (and now OE-Core) require support for RPM? Specifically because there are large segments of commercial customers who specifically require rpm formatted packages in their designs.
(LSB also used to recommend RPM as the package format...)
There is no intention within the Yocto Project or OE to specify one format over another, only to ensure that RPM is on equal footing as to the other formats and not "ignored".