From a features point of view the Pro3 is of course lacking slightly when compared to the Extreme4. Outside of the smaller bundle, the Pro3 doesn't offer SLI / CrossFire support, it lacks the Marvell SE9120 chip that pushes SATA3 from just the two Intel ones to a total of four and motherboard I/O connectivity is also a little less lacking.
It's those extras and a couple of others that make up the $70 price difference between the Pro3 and Extreme4. For some people, though, the extra $70 and features that the Extreme4 offer may not make it a better board. If you're not going for a multi-GPU setup, don't need more than two SATA3 ports and don't need connectivity like DisplayPort, 1394 FireWire and e-SATA at the back of your board, then they're features you're paying for and won't use.
That's where the main appeal to the Pro3 is. Out of the box it's going to offer pretty much identical performance to the Extreme4. Assuming both boards are overclocked to the same speed, you'll also see very little difference between the boards.
The Z68 Extreme4 from ASRock for $199.99 is a great board at a great price point, if you don't need all those extra features, though, and instead would prefer to save some money and perhaps throw that $70 into some more RAM or maybe even kick your VGA card up another model, the $129.99 Pro3 is a fantastic option.
Compared to boards priced at a higher price level, the Z68 Pro3 lacks some of the features that you'll see on them. Not everyone needs them, though, and the temptation to save a good chunk of change will be hard to resist, especially since out of the box performance is near identical when compared to more expensive offerings.