Where I find an electronic load essential is in loop stability testing for a SMPS. With a variable supply, electronic load, function gen and scope you can quite quickly un-cover resonances in the control loop (that lead to sudden loss of phase margin) that clients have tested using power resistors and declared perfect. At the moment I'm still doing it all by hand, but I'm part way through writing the software to automate this process.
This sounds interesting. How do you do it? Using the function generator to control the electronic load and then doing some slow frequency sweeps while testing the regulation with the scope? Is square wave sufficient for the load or do you need any other waveform? Why a variable supply?
Sorry I didn't explain myself well.
The function gen is used with an isolation transformer and used to inject a small signal into the feedback loop. The scope is then used to look at either side of the injected signal, which effectively gives you the open loop response which you can produce a bode plot, or just look for the gain cross-over point and measure phase margin. There's some very good detailed application notes out there on this (TI maybe?), I've got a paper copy of one here somewhere that I'll try to find. Effectively a poor mans network analyser, but it is surprisingly effective.
However, to get a true picture you need to perform this analysis at all speced input voltages and load combinations (and well into overload). Often it'll look just fine, but then just sweeping the load and/or input voltage can uncover a point where the phase margin disappears. Obviously it's impractical to test all these points by hand, but a reasonable procedure is to start at no-load, find the gain cross-over point then start sweeping the load and voltage to see what happens. By repeating this at a few points it becomes apparent quickly if there's any dodgy points.