LOL. "Buy extras because they have specialized parts and when they go down, you're screwed." That wouldn't go over too well where I'm from. If the units are under Dell/HP business class next day business on-site warranty then I suppose that's fine, but most of my clients want ME to service them, not Dell/HP, and some of my clients scream if they're down for more than an hour or two.
Is it worth it to be down a few days, or have to buy extra computers in case one fails just to save a bit of space? Not to my clients. I've even had clients redesign their offices with full sized towers in mind because they want the flexibility of having a fully serviceable, full sized tower.
The only reason Dell/HP design proprietary crap is because idiots keep buying them. If people would refuse to buy crap that isn't serviceable with readily available off the shelf parts, they'd cut the crap and start offering that. But right now they have absolutely NO incentive to use standard mITX motherboards and power supplies because it's cheaper for them to make proprietary crap, and people will buy them anyway. In fact, they'll buy MORE of them just like you do for spares. It's a win for Dell/HP but a lose for your clients. I REFUSE to support bad design and companies that intentionally make things that aren't repairable.