Ah, one of these again.
I wrote a simpler one a couple of years ago (http://code.google.com/p/spawntool/) myself. All it does is read commands from stdin, one per line, and keep a desired number of processes running until all command lines are exhausted. Simple.
I wrote my own because I got tired of all kinds of substitution and quoting issues with xargs. With spawn I only need to generate the shell commands and instead of piping them to bash I pipe them to spawn. Also, this means I can easily review your command line generation with less (so that quotes etc. are good) until I eventually switch to sh or spawn.