I connect my POD X3 to SPDIF.
If I have a quality pre with SPDIF (like MOD BOD) I would probably connect that.

I use S/PDIF for a 24 ch. digital mixer which I have most of my outboard synths connected to. Ya...too many synths....well, not really. No such thing as too many! 

My most used input via Yamaha 01v, I also use the 01V for monitor control so using both in and out. 

i'd like to use the spdif input to handle my wk-7500 stereo out so i can record its midi but i can't seem to find an analog to digital converter 

this would save me unplugging the guitar and mic the ones i've see seem to be digital in to analog out or am i missing something

Just be careful connecting anything to any SPDIF input. Dave is very correct when he says this:

Caveat #1: if you don't set your interface's clock source to S/PDIF, you may get clicks in the recording. 

The moment you connect something there your interface is going to detect the incoming digital clock. It may ignore it and keep working but you could get clicks and spitting sounds a result. My Yamaha digital mixer for example refuses to work altogether (drops into silence!) until you lock the mixer to the incoming SPDIF digital clock.