5 Bikes here (and considering a 6th)….
I would say they all have their place and teach you different "transferable skills"…
I do notice the diffeence but often it's a positive, if I ride my local trails on the DH bike one day and then my XC/Trail bike the next I find I attack more on the XC bike, and ride it that bit harder into corners and off drops, where I'd maybe back off and tell myself it's not the right bike for ragging…
I couldn't use my XC bike for the kind of DH riding/Racing I want to do and I couldn't use the DH bike for XC or trail centres, so they both meet a need
The BMX makes you realise just how much an MTB compensates for your lack of finnese and precision, but I probably don't ride it anything like enough…
My road bike is Just an Efficient tool to get from home to work and back again, but it does help with fitness and make you feel like your getting the miles in at least…
If I had to have one bike it would be something along the lines of my XC/trail bike, a 4" forked HT with sensible gears and brakes, capable of most things, but not quite everything…