On the go moisture testing is a tool.
For those working with a predictable humidity environment it is not really valuable. When I baled at night you could tell by the feel of the air when the hay would bale without shattering leaves.
Baling on the humidity for day time baling, here, is a different story.
I started using a their hand held meter when that was all they had, and it had a dial read out
http://www.delmhorst.com/products_hay.html
I order directly from the main office. If starting over I probably would go with their meter designed to be used as an on the go meter.

From Delmhorst I later bought the kit to give on the go readouts. The wire from the meter to the pick off is simply a CB radio cable. Perfect fit on the meter end.
I understand they now have a star wheel sensor system for large balers and RB balers.

As has been pointed out, a too loose bale will read low and a too tight bale will read high. When the moisture changes I get off and check the bale weight.

I use a C clamp to hold the read out button on while baling, using a meter designed for random sampling. Probably for more than ten years I held the button on by hand when in doubt or curious.

I have never used a digital meter, but have for 20 years watched the analog meter hunt sometimes with each fresh charge of hay.

As of now, I use a Kestrel 3000 hand held weather station from page 379 Spring Gempler's Master Catalog. This tells me the humidity down at the windrow. http://www.gemplers.com/

Gempler's, also have moisture testers also but not what I use.

I agree an electronic sensor is subject to inaccuracies. It does not measure, the full impact of, stem moisture. For that you need one of the heated oven style moisture measuring systems. Problem is by the time you know what the hay moisture was it has changed. The University people with a flock of student labor have no problem.
Individual bale testing or windrow testing is also subject to sampling variability, some call it error.

Irregardlessly, operator technique is important. This includes counting strokes per bale, listening to the slip clutch, the engine noise on each stroke and a lot of other minor hints.
My system is at sunset before the day of baling my hay is a Dry, stem snapping dry. The only moisture I need worry about is residual dew and or relative humidity at the windrow.
I do not leave hay packed in the chamber so there is no chamber rust to clean out, but there is still enough oxidation of the white steel to effect bale density at start up. So it takes me about 10 bales to get everything baling as it should be at 18% to 20% moisture with a an occasional hint of 22% moisture. Then after 20 minutes of baling, if not before, I again check bale weight and length. After that I check weight and length by counting strokes and watching hay moisture. With a heavy cutting of hay & before gettng old with stiff with hurting joints I could jump off check the bales while the baler was still baling. Now I stop the tractor and take considerable time getting down and walking to the back of the baler.