I guess that most people use them 
similar to myself 
I carry one in my wallet, just slap it on a reader and 95% of the time, there’s your answer.
The xFDs 
on the other hand, I dont carry keys at all, ∴ I don’t carry a keyring, ∴ I dont carry xFDs in my pocket.
I do however carry the xFDs in my backpack, and they come out if I’m struggling with a reader and the orientation for a xSeries.
Although I would like to say the DT Diagnostic card SHOULD be the best on the market, in my experience, it is not quite as good as the Proxgrind RF Field detector, they perform similarly, but the difference being, I have found IN MY EXPERIENCE the Proxgrind RF Field detector will “see” the frequency on some readers where the DT diagnostic card maynot, but never the other way around.
HOWEVER
Because of the form factor, this again lives in my backpack with the xFDs, because the DT Diagnostic card, is more than good enough for MOST situations and fits in my wallet, which I do carry with me. Plus it’s FREE
Here’s an uncommon, but still valid example of a Proxgrind “win” over a DT Diagnostic Card