Good question. I used to sell AS/400's, which aren't quite mainframes, but fit the typical picture in your head (green screen, dot-matrix printers, high price t

Good question. I used to sell AS/400's, which aren't quite mainframes, but fit the typical picture in your head (green screen, dot-matrix printers, high price tags and all that). 1) Stability. That stuff changes VERY rarely, and when it does it tends to be very backwards compatible. Our software had binaries compiled in the 80's still fully operational. 2) Reliability. You don't reboot the server. You don't wipe it and reinstall it. You don't even upgrade it or replace it that often. You can do things like add or remove drives from your RAID array while the machine is still at full utilization. Heck, IBM AS/400's will detect the error, order the part, and get an IBM guy there in the morning to install it. You just have to let him in the door. In fact, you can hot-swap things like the RAID controller card itself, which is WAY not something you can do on x86. They last forever, too. It wasn't uncommon for us to have machines in daily use for 10+ years without a single bit of maintenance or an IT guy even looking in its direction. This is for a business critical machine, mind you. 3) Single vendor Ever buy a Dell or HP server and have an issue, and you spend 2 weeks trying to get HP/Dell vs Microsoft vs some other company to admit fault and fix the damn issue? Then you find it online yourself because that's nuts? Well, IBM is responsible for hardware and software on these machines (besides what software you put on it) so if it isn't working, you call them and it's their damn problem to get it sorted. Kind of like Apple, except they'll send as many people as necessary for as long as necessary to sort that out for you. 4) Entrenchment It works, why bother changing it? It doesn't even slow down as it ages, either, since you aren't upgrading a GUI OS like Windows or whatever.