I can explain based on my experience, others might see it different... With the BPM4 running two or more pumps, your fuel demands are now 100% DME controlled, both pumps always running, just at varied PMW based on demand. The wiring and circuitry of this is much more robust than a OE EKPM so when powering high output, high current pumps, you have peace of mind knowing there won't be heat issues with the EKPM.
With a hobbs or JB4 triggered second pump, you have the cars' DME trying to supply X amount of fuel, relay triggers second pump which is literally going from 0, right to 100% DC, now the DME scrambles to modulate the primary pump to whatever output that will bring fuel pressure in line. The DME handles this pretty amazingly, considering the sudden, unregulated fuel flow increase, but it can't match the consistency of both pumps always being DME controlled all the time. My two cents is JB4/ Hobbs absolutely works for most situations, however, to do it the "right" way, and to have OCD level of control over things, the BPM4 has been good for my use.