Many of us use those expansion slots about halfway through the life of the machine in order to upgrade them inexpensively (like adding SATA 3.0 to a machine purchased when SATA 1.0 was still new)
Yeah, because not having SATA 3 makes a machine ununsable...
repair them when a specific component goes tits up (The NIC died? Thats a $15 card for full-on b/g/n wireless)
Yeah, because if a NIC dies then it doesn't matter that it's probably part of the motherboard chipset and all of a sudden you have an expensive repair bill. Not to mention that wireless cards *are* plug-in PCIe cards on these mini form factor PCs...
or to add specific functionality that only comes standard on much more expensive machines..
I think you might be havign problems understandnig/empathising with what a 'normal' person is going to want out of a PC. Not only that, but even us geeks get bored of the PC treadmill after a while. The last desktop I bought was an iMac in 2007. It's old, it's tired and it just stopped working reliably this past week or so. I cant' easily fix it, but you know what, I got five solid years out of that workhorse. I don't care that I couldn't upgrade it -- despite being a programmer, and therefore as much of a 'power user' as anybody, 2GHz of Core2 Duo has been plenty fast enough -- and, if repairibility had been an issue, I'd have bought 3 years of Applecare and retired it when the warranty expired.
The MacMini is an excellent computer if you want a small, silent machine that keeps out of your way. If you want a toy to upgrade and play with, it's not so great.