Re: I'd like to have an honest non-marketing answer to the question...
It is incredibly hard to find non-marketing answers to this. IBM and CA have one set of answers, everyone selling x86 have another. Mainframe techies love them.
My own answers based on reading around are;
1) Reliable - they are designed to just keep ticking
2) IOPS - they handle really high volumes
3) Security - new-ish but they do incorporate in at fundamental levels
4) Financial arithmetic - they are optimised to add decimals (not simply integers) correctly
Everything else relies on believing someone about cost/performance and, since IBM can dial that up and down at will for every contract, I'll leave it out.
I'd like to hear a good answer to why Linux on z rather than on another platform?