Honestly, in the past I've wanted something like this when a co-worker was running a process that was outputting progress, and I didn't want to keep bugging them to see how far along it was. You could use it to check on something like that.
The evil use case is just to watch what someone else is doing.
Is Ispy intended as an alternative to, for example, running tail -f on a log file? Or for interactive programs where I might not have a log file?