I agree that after you know what to look for you can often avoid looking at the details. But how do you communicate that look, that gestalt? That’s hard. Often you need the details to divide the plants into the groups that you then learn.
Also, easy though it is to separate Bromus hordeaceus from B. diandrus, other brome species complicate the picture. Bromus hordeaceus resembles B. secalinus and B. commutatus which are even harder to tell from each other than they are to tell from from B. hordeaceus. B. japonicus and B. squarrosus are really hard to tell apart (lemma margins help a lot). Bromus diandrus looks nothing like B. commutatus but only size distinguishes it from B. sterilis. (All the bromes mentioned in this paragraph are common annual weeds; you can’t argue that you only see one of a group of similar species.)
In a big complicated group like Poa (bluegrasses) there are some gestalt differences – Dr. Robert Soreng, world Poa expert, can often ID photos that my colleagues and I can’t tell apart without seeing the little details! But since we don’t know understand the gestalt differences, we are happy to find any non-overlapping difference in anther length, leaf sheath closure, hairiness of the lemma keels, or other technical difference that allows us to make the call. As we begin to learn gestalt differences, we remain happy to have these details to fall back on for individuals that don’t seem clearly one or another species.
I work on fine-leaved fescues a lot (Red Fescue and the dozens of species that have been called Sheep Fescue & some others). To ID them, not only do I measure lemmas (and this means I need to know what lemmas are) but I cut cross sections of their leaves to see the arrangement of fibers (sclerenchyma bundles). Do you think I do that because of some mad attachment to leaf cross sections? Or a wish to exclude others from fescue ID? No. It takes time and it’s barely possible to do in the field with sharp scissors and a hand lens. I look at leaf cross sections and other people do it too because that’s the easiest way, pretty much the only way, to distinguish some of the species. I wish it weren’t so, but it is. Yes, there are gestalt differences between some of the taxa, complicated by maturity of the flowering branches (= inflorescence) but if you see many of these species you come to prize the technical differences. Clarity! (Relief!)
Some groups of plants are easy. Some become kind of easy after you learn them but are hard to learn. And some are irredeemably difficult, technical. I wish all groups were easy and non-technical! But I don’t get what a want (again!). Sometimes one has to delve into the technical stuff or just leave the observation at “spider” or “slug” or whatever. Unsatisfying but real.