I work in corporate I.T. myself, and quite frankly, I'd be ecstatic if employees here started asking for Macs and we got permission to issue them!
When purchasing, you're typically asked to go get bids from 3 different suppliers. Plenty of people will sell you a Mac. I could go to PC Mall or PC Connection for example, or Micro Center or Insight. Any number of authorized Apple resellers might be willing to give me different offers on a corporate purchase - especially if it was a large system order. We even have a local Apple reseller in town (Mac HQ) who would do such a thing, probably even throwing in free delivery and setup if I wanted it.
And trust me, no medium to large-sized business I know of cares how "serviceable" a design a computer has, any more than they care how serviceable the design is of the microwave oven or mini-fridge they bought for the break room.
They care about overall reliability statistics, which means every time one of those Windows PCs gives a user problems and it has to be re-imaged or have malware removal run on it? That's a mark against its reliability. Macs are generally pretty reliable computers from the hardware angle too. Like anything, you can find exceptions -- but there's certainly no reason I'd believe the average iMac, Mac Pro or even Mac Mini would experience higher failure rates in hardware than anything else I might buy for the purpose.
To be fair, "lack of roadmap" is something that irritates software developers. If I worked for a company that coded and supported its own custom software, I'd definitely take the argument into consideration, if the developers were complaining about it. For everyone else? I think it's a bogus complaint. Apple currently uses the same Intel CPUs everyone else does, so most delays and expected release dates for new systems revolve around Intel's roadmap, utlimately. If you're worried about more details on the smaller changes? Keep up with sites like this one, and you'll know as much as anyone about what's LIKELY to happen next. It's not like Dell or HP gives me advanced previews of new models, many months before they're ready!
The three suppliers that you mentioned will offer you the same price mandated by Apple. It's very different from pitching, say, Dell, Acer and HP against each other.
And if, say, you are an IT manager in charge of buying Macs how exactly do you do it? You wait for Apple announcement to learn the options for the latest Mac, then wait until 3a.m. (or whatever - depending on you time zone) and rush to Apple.com to order your hardware because otherwise there will be unspecified delivery delay. Then you struggle connecting your Apple (TB only) monitors to your PC boxes (just an example), then you run with your Mac Mini to genius bar to replace dead hard-drive in Mac Mini. Is that what you want? Let me give you an example. I work for a large corporation. We have all kinds of special software on our laptops. It monitors "health" of our computers (defragments disks, cleans up registry etc.). This software is monitoring status of laptop battery. When the battery degrades, I get a notification message that invites me to go to PC service department where they replace the battery in 3 minutes. Obviously, I like using docking stations with my laptops. The list goes on and on...