I have a MBP, iPhone and iPod touch. Also I have a semi working 3rd (I think) gen classic, and my parents have an iMac.
I've got 2 drives in my mbp (work and personal) and if I had enough money/was a permanent employee I'd have 2 MBPs for this purpose.
iPhone is obviously my phone.
iPod touch I've had for years (long before iPhone) and is now becoming slightly redundant. I had it in the car as my cassette adapter made those horrible data sounds when plugged into my phone, but I just wrote off my car and I'm putting a raspberry pi with airplay in the new one, so the ipod will probably not be used anymore, or maybe for long journeys (battery) or situations when the device might get damaged (running when it's raining)
Finally the iPod was a hand me down/scavenge, not used as the battery isn't up to much. Might get round to doing something cool (but useless) with it someday like putting it into some obscure room/appliance in the house 😛
On the multiple macs front, I in the future can see myself having 2 MBPs, work and personal. Also for people whose work is maybe more integrated into their personal life (e.g. photographer) I can see why they might have an iMac/pro for power and an air or mbp for portability 🙂