I'm not suggesting using "Internet Sharing" per say. Internet Sharing is a dumbed-down version of simple routing for those that lack true server software. The Mac mini would be almost a perfect replacement for a router that requires only wireless access (precisely the setup I use at home, btw). You get much more fine-grained control over the "sharing" of the internet connection when you use true routing software (as is included in Mac OS X Server), such as control over DNS, DHCP, SNMP, firewalling, spam filtering, traffic shaping, QoS, etc.
I don't use a single ethernet port on my Airport Extreme other than the WAN port. In this light, if I required finer control over my wireless network, I could replace my entire router with a Mac mini without losing a single feature, and gaining quite a few more features (such as UNIX permission-level control over file shares, managed FTP access, web serving, etc.).
I understand that a network that requires wired clients may not have as much a need for a mini as other networks, but that doesn't mean that the mini wouldn't have its place among bigger, beefier servers as well.
I have to agree with you, if I may mangle your words a bit, that the Mac mini would not make a stellar, high-throughput gateway... but it would still make a mighty fine server if your needs don't require saturating a gig-e network with mountains of traffic.