I hear your pain...

The issue is that all of these sizers are slightly different. And the sizers are kind of useless if it comes to the space that's really there (depending on the aircraft).

My take is that if you know what you will fly (e.g. Delta connection on regional jets all the time etc.) then a special bag makes sense. If you fly a lot on mixed equipment, I say buy what works for you.
I only experienced two "discussions" about my carry-on last year (out of about 40 international segments) and both were not about size but about weight. Nothing I couldn't explain and resolve within 2 minutes or so. Granted that was mostly in C but still the argument of "why are you strict on my baggage if I save you time and effort by not checking anything?" usually works (and I'm nice about it, and stay size wise within the limits).

I think that psycology really is a big part of that: Come with some bags that look empty and you are good even if they are an inch bigger than allowed. If the bag looks stuffed you'll be checked.