"Not to be the one to turn down a gadget on a tractor, but this type of gadget seems to be nearly useless"
Not at all!!
As R&B notes in their literature, the slopes that increase (starting off mild, then increase in steepness) can be very deceptive. This gauge...(a guide, as noted in another response) can keep you from getting into trouble.
In my case (and if you read the posts of several other "newbie" tractor owners), I thought some of the side slopes I was on were horrendous..turned out to be well under the limits of safety. As I used the tractor more, I felt a heck of a lot more comfortable.
Initially, I was doing the whole yard in first gear Low. Now, I'm doing the majority of the mowing in 4th gear Low, which is probably as fast as I'll get on my sloped property. 4th gear Low runs about 2½ MPH.
Part of this is, of course, the learning curve. But I attribute much of my comfort to knowing the approximate angle I'm driving on.
Now, the use of the tiltmeter is not to replace common sense. And, as another respondent to this thread noted, the limits of the tractor can change, dependent on how it is equipted.
In conclusion, I must say the tiltmeter helped me in my learning curve and is considered a valuable addition to my tractor.