When I started birding in the '60s it was considered a grave faux pas (within my circle of birders at least) to be caught using a field guide to identify what was before you. The logic was that you used the field guide to bone up on bird ID before you went out into the field so that you could confidently identify most (if not all ) the birds you saw. When faced with something you couldn't ID there & then, out came your notebook into which you put your sketches & detailed notes. Only when you had exhausted your descriptive powers (or the bird had flown off) was it considered OK to check in your guide (if you had one with you which wasn't always the case). The method taught you how to look at a bird systematically, helped to give you an understanding of different bird families and avoided the temptation to 'fit the bird to the picture'. If it wasn't in your description then it wasn't there!
Roll forward to the present day and I still seldom use a field guide to ID birds to species level in the UK & Europe thanks to the solid grounding these methods gave me in the 60s & 70s. (But I seldom use a notebook these days!) There are some exceptions with sister taxa which are almost identical (esp. recent east/west splits). However, having a guide on my phone is a great temptation. I sometimes use a guide in the field in the UK/Europe to decide the sex/age of birds and check the calls of the increasingly few birds I can still hear. When I'm outside Europe (which isn't often), I'm still loathe to use a field guide if I can avoid doing so by learning salient ID features of the birds I anticipate seeing. However, my capacity for learning & retaining information isn't what it was so I always have a field guide handy as a back-up in such circumstances. It doesn't help either if the bird families are radically different. I still suffer a scintilla of annoyance, though, when I have to pull out the guide.