I've found it most useful to track down shorted components, and once to find an extremely elusive short on a PCB that was poorly manufactured.
The method I use is to inject 1kHz on the shorted signal (typically the power supply) using a small signal transformer, and then use the iProber to track where the 1kHz current is going. The small tip of the iProber allows you to get down to the exact pin on a 0.1" DIP. You can easily identify the ingress and egress of the current through the shorted component.
If that's your main usage then surely all you need is a tiny coil, and perhaps an amplifier, for tracing an AC signal - for very little money? The question then is can you justify the rest of the cost for the less common use cases?
Sure, you could build something that would do similarly. You could also buy an old HP 547A current tracer probe for less than an iPober.
We had an opportunity to buy an iProber as part of a consulting contract to assist in tracking down failures in a third party, battery-powered energy metering device. The meter was encountering a number of ESD induced failures. The major failure mode was CMOS latch-up on various parts, so it wasn't as easy as finding a dead short.
The iProber made it very easy to identify which chips were involved in failures, and down to the pin. It was a board of about 20 SOICs (0.05" spacing), and a microcontroller with even tighter spacing (I think it was 0.65mm). That might be difficult to achieve with a home brew coil, especially on a tightly packed board. And add to that repeatability for the relative value, since it's not an "all or nothing" measurement (there's other legitimate current flowing besides the fault current).
The time saved in not having to build something, and the ease of using the iProber with its long insulated tip was justification enough. It got the job done.
In the case of the voltage regulator I mentioned, there was a problem with it going into dropout for 50us which was causing a board to reset. At first it appeared to be the voltage regulator was not up to spec. It took about a minute with the iProber to figure out it was 1 of about 15 chips that was putting a huge spike on the power rail (it was a bad driver chip). And without cutting any traces.
The iProber saved me time here too, but of course there's plenty of other ways to figure out this one too.
The iProber is handy to have around, and I use it now that I have it. Without a specific purpose, my justification would be as a convenience. It's certainly not a necessity.