nickvd,
I'm happy to explore other ways to pull that off. There are a couple of reasons for having the local circuit portion of the module -- and yeah, they are all electronics related.
The capacitor in the sensor local circuits is acting as a noise filter, and it needs to be located physically close to the Arduino to be effective.
The other thing is that by keeping the power line in the brain, you only need two wires out to the sensor (instead of three).
The idea is that the module contains all the pieces it needs to work. So one soil moisture sensor needs one local circuit -- the whole thing together makes it give you a signal you can use. If you want to add a sensor, you will need to duplicate the local circuit portion as well. And build it this way helps you understand why the sensor works so you can make adjustments.
I like the idea for a super easy interface. A note, I had trouble running the LCD display at the same time as the sensors -- a ton of noise! That's what pushed me to do the charting on the computer. But if you have interest, I say go ahead and start playing. And let me know what you find out.