Average for selected text could be nice. But truly I was thinking averaged out for a given and selected Text Frame. In practice, I set up a text frame and then want to make it readable by getting the sense of the measure of the paragraphs within that text frame. Most often, I make adjustments either by changing the width of the frame or adjusting the size of the text. I never really care about the paragraph measure on a single line of text, only in blocks of text because I usually care about paragraph measure in relation to readability of a lengthy text.
If I could quickly see what the paragraph measure was, on average, for all the paragraphs in a given text frame, I'd know how wide to make the text frame and/or how I might adjust the size or other characteristics of the type.
I think having it display in characters or words is fine either way. I use words and I suspect most people who actually care about paragraph measure do as well. But characters is rad too and I could learn to adjust. Or it could be a setting somewhere.
I would see paragraph measure as an attribute of an individual text frame (a function of typeface attributes like size and the width of the text frame itself). There may be several text frames on a page, each with different measures (main text and sidebar text, for example). I usually think of word count or character count as attributes of an entire document. The average paragraph measure of an entire document wouldn't be very useful for me, I don't think I would ever use it to make design decisions. But I would use paragraph measure to make design decisions about individual text frames.