This response is from Betty Azar
In saying "I intend," you are saying that you have already made the decision to do something. It is the stative meaning of the word: the speaker is saying that this intention is a "state" that exists now, with action of creating the intention having taken place in the past. There is no action taking place at present. The same is true of "I plan." I really don't know what else I'd say.
Their use in the present progressive is akin, it seems to me, to the progressive use of "think" -- this is what's going on in my head/mind/soul right now.
In other words, "intend" and "plan" have both progressive and nonprogressive uses -- exactly as you say.
The reason "plan" and "intend" are included in this exercise is that a fairly common error among learners in using these two words is to use them with "will" -- e.g., "#I will intend to quit smoking." In terms of text organization, the dialogue that these two verbs occur in needed to follow the study of the present perfect as well as future verbs, so it occurs in Chapter 4 rather than Chapter 3 (Future Time) -- but the real intention is to point out that these words talk about future events but are used in present tenses to do so. I hadn't considered that progressive vs. nonprogressive uses could be a problem here.
In the third edition of Fundamentals of English Grammar, I changed the term "nonprogressive" to "non-action" with the intent of simplifying terminology. "Nonprogressive" seemed like a mouthful, and I didn't think defining a "stative" verb should be the focus here -- but covered these bases if teachers wanted to use this terminology by including the two terms in a footnote to the chart. I admit it is an imperfect solution, but I continually seek to simplify terminology and try to keep the focus on the form and meaning of a structure. Terminology is just a transitory tool to aid teacher-student communication. Whether these verbs are called "non-action" or "nonprogressive" or "stative" is not something that will stay with the students, and the teacher should use the term that suits his/her teaching the best. But it's important for me to know what terminology causes problems for teachers -- so the input is much appreciated. I'll certainly reconsider the terminology during the next revision.
Thanks for commenting!