Giving up the guide in the field can be seen a couple of different ways.
One is as a matter of individual development as a birder. It means saying that we know the species where we are well enough that we will quickly recognize almost all of them -- and if we don't, we are skilled enough to know what to look at, what is different, how to look at the bird and study the field marks well enough to allow a later definitive ID. Getting our knowledge to that point -- if that is our objective, is a profoundly good thing on a personal level, something to be proud of.
Another is as a social and status thing among birders, kind of helping to establish your place in the pecking order so to speak. While the Brits are quite different from here (and most of the rest of the world) in many ways with birding, this is also something I see here. Pride goeth before a fall however, and every year as predictable as the change of the seasons, several of the self-designated first tier birders, the kind who would never be seen consulting a field guide in the field, will make dodgy calls that just might have been avoided if they had a field guide in their backpacks.
Personally, a field guide went with me almost everywhere for years, and one is still right by me whenever I go off my "territory". I have little patience for the hierarchial quirks of the local "elite" birders, and have no problem with openly carrying a guide in situations where one is desirable to me. Nonetheless, I never carry a guide anymore for my walks in my local patch, because I feel it is the right thing for me, and where I am currently at with my hobby.