I think that there is an interesting question about an application like SheepShaver. PPC Macs are going to be around for quite a while, and Classic apps will only make them more valuable as the newer apps outclass their capabilities. So, the options would be to limit a PPC Mac to <10.5, or to have something like SheepShaver.
I, too, am curious about the use of a virtualization option like Parallels to access a Classic environment. But I doubt they'd jump at that option as they wouldn't have much of a market unless they created both virtualization and emulation modes (one for the PPCs out there and one for the MacIntels). And suddenly we're looking at a whole lot more work.
Now, what really has me curious is the whole question about PPC ROMs with SheepShaver. Okay, I have a Mac that could run Classic (before it was cut off), why do I need to try to track down a PPC ROM to be able run SheepShaver? This is what I see as the great impediment to SheepShaver being an effective option for those of us who want to run Leopard but still have access to our Classic apps. So, I'd see this as an issue that needs to be sorted out in order for SheepShaver to appeal to the masses more. Hopefully, with the dropping of Classic support from Leopard, some new blood will flow into the SheepShaver project.
Why? Because, as others have pointed out, there are apps out there that have no OS X version. Sure, with some of them there may be an equivalent program, but that means buying something new, learning something new and translating data. If the app in question is only rarely used, but still necessarily used, that's a pretty big investment to make if a simple means of gaining access to the old app can be found. Further, this only covers some of the apps. There are apps that simply don't exist for OS X. Perhaps most notably, games. You can't simply buy an upgrade of a fifteen year old favorite game. And keeping an extra computer around just to play it seems excessive if, again, there can be a simple way to maintain access to them.