I'm using SPDIF all the time in my little studio, both ways, i.e. for recording of dry & stack or master mono signals as well as for reamping afterwards. In many cases I take the analog ins of my audio interface for special purposes like a non-cab signal for use with IRs in the DAW or exactly as you say it above to record the stereo effects of the Kemper in order to use them in the mix (here mostly the incredible reverbs).
Advantage of SPDIF? No additional conversion of the signal from digital to analog and back. Others here might comment that with a decent interface with decent converters in it you won't be able to hear the difference. I agree with this. If you have a crappy interface it could be a different situation whereby the crappy ones certainly don't have SPDIF then anyways.
Can't comment on your optical-thingy converter. But what I am reading here in other threads that can work but some products are not optimal and create a bit of hassle.
So overall: I love the SPDIF stuff, it works flawlessly, easy and convenient in my situation, gives me up to 4 tracks that I can record from the Kemper at once and all the flexibility along great sound
Thanks! I'm using the Motu M4 (4 inputs). The TR analog sounds fine to me but I may want to go the SPDIF route too. I figure not much of risk to try the optical-rca converter. I should be due to build my next computer in about 10 months or so - I'll make sure the sound card has an rca SPDIF on that one.