That's an expense rack. If the average MBP cost $1500, that's a total of $144,000 in the one rack. They must have some serious reasons to create something like this.
Are there good reasons to believe that "enterprise" hardware is more reliable in practice?
Anecdotally, most desktops I used lived much longer than most servers I worked woth. I know a Compaq desktop that worked as a server for 11 years. Still does. One hardware failure in that entire time. (Power supply.) Could be a result of lower workload, but still...
There are superior aspects to the design of "enterprise" hardware (or rather, rack mounted server hardware) that have been mentioned by others: ECC RAM, redundant PSUs and HDDs, motherboard designs that (hopefully) are laid out for ideal airflow, etc.
Putting all of this aside though, the real secret to the superiority of hardware meant for the datacenter often comes down to the practice of binning. Manufacturers have different tolerances for the products they produce; hence why Intel has a million models of CPUs, Seagate produces so many different hard drives and Samsung sells DRAM chips to other vendors and makes their own DIMMs.
The products that perform the best are binned in the server-y bins, the others are moved down the list until they fit another bin. No manufacturer wants to discard parts if they can possibly avoid it.
Sometimes you're actually getting a deal, but a lot of the time you're just trading reliability for cost when you use lower binned items like desktop hard drives in a server environment. Sometimes that trade-off is worth it though.
Sure, but we're not speaking about overall system reliability. We're speaking about hardware dying. RAID will prevent data loss, but your actual disks might fail as often as something from vanilla PC.
The refurbished machines are a decent deal and I haven't had any problems buying them for personal or business use in the past, but the big limitation is on supply at any given time and the configs offered. Refurbs are largely stock configs and you can usually only buy 5-10 at a time. There are big benefits to config uniformity and bulk purchases, so refurbs can be tough in that respect.
Also, no one buying in this quantity should be paying list price to Apple, which cuts into the refurb discount considerably (if not outright eliminating it).
$1500 is the entry price for 4-way xeons, and that's before you factor in the chassis, motherboard, RAM and PSUs. You can easily find 4U servers more expensive than that, the rack seems to have a 32U capacity and could fit 8 such servers.
A standard server rack (fully populated, just regular app or web servers) can easily be 400k+. Everything built to run in a datacenter is pretty expensive, even if you've done your negotiating work and aren't foolishly paying list.
With such a large capital expense, it makes a lot more sense to rent dedicated mac hardware from a data center. You get simple monthly bills, guaranteed uptime, and the infrastructure is looked after for you.