Ahh yes, you are correct, it's only about 10 bucks cheaper; I was looking at the module price, not the price of the whole package. And much larger, but also supporting a wider range of logic levels and providing better overvoltage protection on its inputs. Also, what gives you the idea that the DPT's RF isn't calibrated?
Face it, your argument boils down to "it costs less and has more features". The only potential downside is for projects relying on "perfect" RF, assuming you are correct about the DPT-board's RF not being factory calibrated; for prototyping, or projects that will primarily make use of the ethernet ports (or not rely on networking at all), it's still a less expensive and better-featured option than the Carambola. If you're far-enough along in your design process that size has become a consideration, you're probably past the point of needing physical ports on your development board, which makes the DPT module (the blue board) about $24 cheaper and a better option; if your project outgrows that, you're far enough along that you're having custom boards fabricated and neither product is an option.
This has its place, especially given the software suite it will ship with. That place is early-stage prototyping and proof-of-concept development, where silly mistakes like overvolting an input or connecting to the wrong pin, which would kill the Carambola, are most likely.
