You can go faster in a harder gear than you can spin-cruise in (high rpm for 2 minutes). Especially if there's any rolling dips and tops. For commuting, a harder gear that you can put in an extra 20 rpms if you need to, is usually very convenient in traffic. A harder gear is also good if you have old/cheap shifting technology where shifting isn't as smooth or non-chalant as it is on brifters.
I agree though, that its great training to mix it up.