Entirely agree with you. That is the singular area that dpkg used to beat rpm as a format. That isn't a feature of apt-get or yum, but of the underlying package format.

However, it is supported in a very new release of rpm:

http://www.rpm.org/wiki/Releases/4.12.0#Generalbugfixesanden...

""" New --recommends, --suggests, --supplements and --enhances query aliases for querying weak dependencies """

So it is coming along :)