[QUOTE=mack]
a minidisk 8-track
[/QUOTE]
Is it the Yamaha MD-8 by any chance? I’ve had mine since '99 or so, and I’ve gotten so much mileage out of it!
As to the OP:
I’m a professional musician (pretty much my sole source of income for the past year, but I still feel weird calling myself ‘professional’), primarily a keyboardist, and my keyboard-specific gear includes (for the most part):
A 76 key non-weighted ROMpler (Alesis QS7.1)
A few rack-mount synths (Roland JV-1080, Oberheim Matrix 6R, Yamaha TX802)
A Virtual Analogue synth (Novation Supernova II)
A small ‘cheap’ analogue synth (Roland Alpha Juno 2)
A monster analogue polysynth (Oberheim OB-8)
A 49 key MIDI controller
As to why I’ve got what I do, some of it is professional necessity, and some of it is for fun.
I’ve had the Alesis board basically forever, and it’s served me reasonably well in a number of band, solo, and pit orchestra situations. A few years back I was doing a show and needed some sounds that I just wasn’t happy with or able to get on that board, so bought the JV-1080 and TX802 to cover those. I haven’t used the TX802 since, but the JV-1080 has become vital to a lot of my work (I use it exclusively for pit orchestra situations where I have to cover different orchestral parts).
The Supernova II and Alpha Juno 2 both get a fair amount of use when I’m doing rock shows (as does the Matrix, rarely), though often I just use the Alpha Juno to control one of the other synths.
The OB-8 just gets played at home. When I bought it I kind of thought about gigging with it, but I haven’t had a situation yet where I’ve felt that the sound I could get from it as opposed to something else I have was so vital that it would be worth trying to fit it into my car.
I’m really very satisfied at this point with what I’ve got at my disposal. I’m trying to make up my mind about one more board though. I’m planning on getting an 88 key weighted board this year so I can really do justice to solo playing (the semi-weighted Alesis board is just not adequate for piano playing, though it’s great for organ/synth/etc stuff).
I realise that I haven’t exactly answered “why” yet. Why is because each piece adds another bit of sonic possibility that I didn’t have before. I don’t have a lot of redundancy in my gear, though I freely admit to being able to get by with less than what I have. Really, the JV-1080 and Alesis QS7.1 are in many ways alike (at least for my purposes). However, the places in which they differ are dramatic, and they both serve different functions. Also, since the 1080 is a rack synth (no keyboard attached), I can bring it to places where they’ve already got a house keyboard without having to lug a whole keyboard of my own in order to get the sounds I want.
The OB-8 is really the one extravagance, but it’s sound is awesome, and the satisfaction of owning a truly classic instrument is worth it. And, I do make music with it at home.
But really, one thing I’ve discovered over the years is that part of getting the gig is having the right gear for the occasion. And, when it’s your job, and your purchases are a tax write-off, you get tempted a little more 
But really, at this point, aside from the 88 key board I’m looking at, I’m running out of serious GAS. I’ve got so much to fiddle with already, and the instruments are so expensive that my desire for other awesome boards is outweighed by my satisfaction with what I have plus my lack of funds.