I've read interviews with Doug Wimbish and Nathan East that they remarked that the five would let them double the synth bass lines that were becoming common on 70's disco and moreso the 80's synth-pop records, and keep them working.
You have to remember that as synths and sequencers and MIDI grew more and more controllable in those days, there was a certain concern in the bass and drum communities that we might indeed, be put out of business by the Linns and TR Boss products. And as the DX7 and that whole wave of much better synths came along in the early 80's, at times it was palpable. More than a few hit records had that big, fat MiniMoog bass.
Over time, in dawned on folks that a good drum machine would do, but it would do MUCH better if it were programmed by a real drummer. Same with key bass, and I even doubled in a few bands for a few tunes back then.
The first 'off the rack' five I remember seeing ads for was the 1st-gen Yamaha BB5000. By the mid 80's, lots of brands offered them, but they were still a bit exotic. Lots of guys dove in headfirst just to get those few extra low notes: I'm sure most of those are still under a bed or in a closet somewhere, after it dawned on them it's a hard row to hoe to completely learn another instrument just so you can play low C and D, just the wrong approach, but who knew ?
Joey