floating ground means.... well..... floating ground, doesnt mean theres NO ground and in respect to portable it most certainly doesnt mean no voltage. When 2 interconnected devices with a shared ground such as with electrical spdif (without transformers) are floating? no they arent; a difference in potential between the 2 ground references (usually in the MIDDLE of the supply voltage, so battery VCC/2) creates a loop.
you can even have 'ground' loops in the one device with a single ground, created by the resistance of the ground plane, higher currents in some places on the plane and ohms law means there is higher resistance->higher voltage drop across some parts of the copper. the same effect can actually create voltage from current loops, current/resistance of the copper = V.
throw 2 different 'grounds' at 2 different voltages and you have a ground loop, it has nothing specifically to do with earth, though earth has its own set of related problems when people use it incorrectly.
I think you need to do some reading, even in AC systems in many cases 'ground' is nowhere near the signal, its just a reference voltage half way in between the transformer primaries, or main reservoir caps, which are also floating. actual ground is only used for safety and in some cases AC filtering and not even always.
yes toslink has higher jitter as an interface due to the conversion to light and back, than a perfectly executed electrical spdif (which we simply do not find in portable devices, mostly not even home devices), but the jitter on the output of any modern receiver is determined by the receiver and its clock, nothing thats any good uses the recovered spdif clock for audio, so what does it matter? in such a case optical solves more problems than it creates.
the fashionable fear of optical, EMI and the trend of thinking i2s is automatically lower jitter are irrational beliefs based on incomplete data/science. i2s CAN be better if over short impedance controlled distances, just as spdif CAN be better if done properly and when looked at in isolation as an interconnect, but if you actually look at the whole system of interconnected devices each has quite rigid tradeoffs. optical has only the one disadvantage, which is taken care of by well controlled and understood devices/designs, the very same ones designed to take care of the issues of electrical spdif.