Whilst it is true, that the average PC, be it home or business rarely gets upgraded, it isn't unknown, that the hard drive, optical drive or the mainboard will fail during the machines lifetime. Especially if they are in a dusty environment - the old slot-loading DVD drives were the worst, a friend of mine is a heavy smoker and his slot loaders used to last about 9-13 months, before they would get too gunged up to work!
Being able to replace those items, on site, with no special tools is really helpful for the standard tech / sys admin. Slapping in a new hard drive, in seconds, and letting it rebuild itself from an image on a server is a lot easier and than having to dissect an iMac, or having to send it off to an Apple Service Centre...
Edit:
How many different stand formats does the iMac have? There is the standard hinge, but that's it. The Lenovo has the standard hinge, the wall mounting telecoping arm and the height adjustable foot.
You can do the HDDs on the iMac, but you either need to dismantle half the machine (breaking the warranty) or send it off to an Apple Service Centre (if there isn't an Apple Store in the area).
If it is like Lenovo's other devices, you unclip the back and unclip the hard drive and clip the new one in... That is a lot quicker and easier than on an iMac.
For a company installing hundreds of these, that is a good argument to go with them, because, no matter how reliable a manufacturer's products are, some will, inevitably fail. If you can replace the faulty part in 30 seconds on site, as opposed to 15-20 minutes, or sending it away to be repaired, that makes a huge difference in the TCO of the product.
Over the years, I've been very lucky, I've lost 1 hard drive and killed one processor, whilst upgrading its cooler. I've had to replace 2 DVD writers, other than that, from the 15 or so computers I've owned, none have died prematurely on me. But we had a delivery of 60 Compaq laptops, back in the 90s, with an 90% failure rate (either dead on arrival or failing within 1 month), even though Compaq won the Computer Shopper reliability award that year, with a reliability rating of over 98%... We just got a faulty batch.