Originally posted by _who_ Why would you need more than one?
I've got:
FA 50 f/1.4 (AF for the dSLR and AF film SLRs)
M 50 f/1.7 (Incredible optics, great manual focusing--used mostly on my manual film SLRs)
A 50 f/2.0 (Not so great, but I'd never had a Pentax-A lens and wanted to try one on the cheap)
Helios-44M 58mm f/2 (Screwmount, used mainly on my screwmount body/MF k-mount bodies, but also on the dSLR. Interesting optical qualities/bokeh)
Helios-44K-4 58mm f/2 (K-mount, used mainly on dSLR and MF film bodies. Incredible sharpness stopped down, very strange and wonderful bokeh opened up)
So as you can see, they all have different and interesting qualities. That said, I'm planning to get rid of the A 50 and probably the 44M, since they're essentially redundant at this point. But they weren't when I got them!