Or the quality of the instrument ie how accurate it is. I agree with Mjit, that in our club cars we change brake fluid quite often, and even with an unknown ie a new purchase I'll change the fluid as one of the first maintenance jobs.
With moderns it's a different story; I've never done a complete change in my Mundano, although when I replaced the brake discs earlier in the year I bled the fronts. I'd guess, as Doug states, that modern fluid is nowhere near as long lasting as the older stuff, which is strange given that very very few drivers touch their own cars these days, or even suspect they might have to change the fluid periodically
I don't think I'll need to buy one of the testers, although if you could use it as a taser by reversing the battery I'd buy one for the cat.