I'm suprised there aren't more than that. It must be mainly used for multi-effect consoles. I'll definitely get one for the PS-5, whenever I actually get the pedal. Any comments on that combination are welcome.
Well, since bossarea brought it up - to complete the topic of external control inputs - the SD-2 and DS-2 allow for an external switch and the DSD-2 / DSD-3 allow for an external input to trigger a sample. The only other external input other than the send and return loops such as the NS-2 and LS-2 would be the HR-2's "detector" input.