...one could just use six 1U servers instead and save a bundle of money and 1U of space, or even put in the new Supermicro 1U servers that contain two motherboards side by side.

Once upon a time the idea of blades was more density and less cost (due to shared PSUs etc).

Now they seem to be sold on the basis of less cabling (because it's all built in) and ... I don't know. I've no idea why people buy them - they seem to be slightly bigger and much more expensive than 1U servers, with no killer advantage that I can see.

Cable reduction is a big deal, especially with SAN. Shared PSUs and fans also make sense. The real point (at least with blades from IBM, HP and Sun) is management and instrumentation that cannot be replicated with traditional servers in sheet-metal racks.

Virtualized I/O, headless, automated deployment, "hot-sparing" all rely on proprietary infrastructure.

VERY good for the vendors who win. VERY bad for the competitors who lose.

Customers give up easy switching between suppliers, but gain a lot from R&D which can only be spent in making vendor-specific features.