For me:
I use my K10 and K20 almost exclusively now. I have three Pentax brand zooms that take me from 10mm-200mm. I also have a couple of the new Pentax primes and many SMC Taks from 17/4 - 300/4.
I like the smaller size and weight now as I move down the path from young whippersnapper "hood" to old fart "hood". It's easier to carry around.
If I need something special, like extra resolution/enlargement or fast wide angles, I use my old Canon primes and cameras.
I am not as young and stupid as I used to be and my back is not what it was. The reduced size and weight of the Pentax system is great.
On my last film only trip (to Rocky Mountain NP) I took two Canon F1Ns, 17/4.0 (104*), converted SMC Tak fisheye 17/4, 24/2.0, 35/2.0, 55/1.2, 85/1.2, 135/2.0, and 200/2.8. All but the 24/2.0 were metal barrels. I remember more of lugging the gear than of the pictures I took. (don't forget the tripod or the 20 rolls of film!)
The IQ of the gear exceeds my abilities most of the time. For 90% of the pictures I take, FF or APS size does not matter, so why not use lighter, smaller stuff.
p.s. I still see my Canon F1N with the 55/1.2 ASPH and my Canon 7s with 50/0.95 sitting on my shelf calling to me every time I pick up my K20!