I only used mine as 2nd CPUs and not to take over the system (they are too slow and cranky for that). As an example I used one with 80MB of RAM on my IIfx to browse the web. You could also run a server in the background (even better if you have the SCSI addon for storage).
They make more sense on a slow 020/030 system with little RAM and no FPU then they do on a PPC. Why bother with 68k code on a PPC machine when you can find PPC native coded apps now.