You clearly have never run servers.
1) An Xserve, even the out of date ones we have currently, are way faster than a Mac Mini. Apple's own figures say that a Mini Server will handle approx. 50 clients, whereas an Xserve will handle 500. That's 10x the performance - so even if you buy 4 Mac Minis, that's only 200 clients to 500.
2) Xserves have Lights Off Management. Minis don't. Xserves have proper hard drives in, not 5,400rpm laptop drives. Xserves have an SSD for boot drive option. Xserves have slots for Fibre Channel cards. Minis don't. There's a mass of hardware differences - oh, and the Xserves can take way more RAM.
3) Redundancy when you're dealing with a bunch of servers is far easier when you can just pull a duff drive module without taking the server out the rack, or pull the PSU whilst the server keeps going on its secondary PSU. Mirroring entire machines is a waste, as you have to have duplicates of every machine, which is a waste of cash.
We have 20 Xserves. 18 are live, running full time. We have 2 spares that can jump in if one develops a serious fault requiring it to be pulled, and we keep spares for most parts in house. Minis just wouldn't handle the job.