2.5 and 5 are based on the existing 10gbase-t technology, and can be deployed over existing Cat5e and Cat6, whereas 10gbase-t only works reliably over existing Cat6 sometimes less than 100ft.
802.3bz's genius is basically running 10gbase-t at half or quarter the clock speed, requiring lower spec cabling but also lower power/lower heat parts. I don't know why this wasn't done as part of the original 10gbase-t specification (802.3an, which is now 10 years old; in comparison, 802.3ab, which defines 1gbase-t, is 17).
In addition, for embedded hardware, 2.5 and 5 are a better fit for common on-chip SerDes implementations when you need to wire stuff up that way.
As for power usage, a modern 10gbase-T controller ran at 1/4th the clockspeed (and voltage also appropriately adjusted) would use less than 1/4th the power, but be more power efficient than an existing modern 1gbase-t, per gbit/sec.
The real question is, when will all the cheap "we need an ethernet port but nothing fancy" controllers (which are all gigabit in anything recognizable as a computer, 100mbit becoming more and more rare in anything not recognizable as a computer) become baby 10gbit? That's all I really care about.
http://www.fs.com/products/30851.html
Same price of $15 for 3 meter. Slightly more if you're scared of buying things direct from mainland China.
A quick search for SFP+ cards that can handle 10Gbit/sec shows that the typical price is ~200 USD/card right now (this is still at least 3x the cost per card I'd prefer to see).
Given the pricing on individual cards I'd hate to consider the price of a switch, if such a thing even exists.