Is there any value to brake fluid testers?
 

Brake Fluid Tester - EBC Brakes

They seem very cheap compared to how much extra money can be spent on just changing fluid because of X amount of time or Y time at the track. Not as good as a true boil point tester but its less then a 1/10 of the price of those.

All that instrument measures is water content. Seems to me that if you're getting moisture in your brake fluid, you have other problems that need to be taken care of. But I'm not a track person and maybe water is a bigger problem than I realize. ;)

seems as useless as someone taking a battery measurement using a voltmeter to determine cca...?

if in doubt, change it out....

Brake fluid invariably gets moisture in it one way or the other, that's why you have to change it out every so often. DOT4 fluid sucks it up faster which is why you change it more often.

Zdayz 2014!!

Yep, but my question is how often is often enough? Every track day, every other autox, every few months, every year, every few years?

I have a brake fluid tester and it does seem to reflect what you would think to be reality as far as moisture content. Older less cared for vehicles seem to have more. Even my dot 4 fluid checks out as about perfect after over 6 months of not touching it. I have no doubt it'd be fine for over a year. Even so, my practice is now to bleed one bottle of brake fluid though the system every track weekend or two depending on frequency. Even this is overkill, bleeding just enough to ensure no bubbles are present on a regular basis should be sufficient. The tester is still a good peace of mind test though.