I had never heard of these, but from the data I'd say their usefulness is mostly in the form of convenience. The exact same kind of measurements have been done before, by many, for decades - I've done it. You measure the DC and low frequency content with a regular scope input, and the high frequency content with an AC-coupled 50 ohm wide BW input. These "power rail" probes just combine the measurements into one channel, and add DC offsetting - look at the 7A13 for an earlier example.
They are intended for low impedance sources, like power supply outputs, to look at interference, ripple, and noise. There is a generalized term for all of this, called PARD. If you want to see really high frequency content, you use 50 ohm wide BW gear - a scope for time domain, or an SA for frequency. The important thing is to isolate the DCV of the supply by AC-coupling, and most importantly, to protect the equipment from surges that occur during connect/disconnect, power up/down, and PS or load faults, etc.
In the block diagram, it may be shown simply as AC-coupling, but in the details you would find that the HF signal path is all RF, 50 ohm environment, including the RF coupling cap, with sufficient voltage rating.(not just any old cap), and various protection circuitry on the 50 ohm output side going to the equipment.
So, if you can live with the inconvenience of using a regular scope input or a 7A13 for DC and LF, and having to rig up a fairly simple AC-coupling and protection circuit (the fanciness and complexity depend mostly on the required BW) for HF, you can save the 4 grand or so.
Also note that the key to HF performance and CMRR is to carry the signals directly in coax, with minimal impedance connections. You can get some ideas looking at the optional accessories and how they attach to the DUT - that's the part that really makes it work.