Efficiency of laser diodes goes down quite a bit with bandwidth. More importantly you typically want the data going the other way (from the powered sensor). If you would use the same fibre for both directions (might be done for space constraints) the issue of using the same wavelength is that there are scattering processes (some fundamental to how fibres work) in the fibre that will cause some light to be back scattered and act like noise essentially. Your sensor would transmit with only very little power so the SNR might be completely destroyed by the back scatter of the high power piwer delivery light. If they are at different wavelengths they can be easily separated.

Oh, you are right - I didn't think about the other direction at all. Thanks for the explanation!

I was thinking that the "power extraction" might attenuate the signal too much, and it would probably lower the power output since you need to modulate the light to transmit data, instead of having it on full brightness all the time. But maybe it would work for certain applications!