Perhaps there are two distinct use cases here.
Use Case 1: your chapter heading already has a distinct class assigned to it that is dedicated just to those headings. In that circumstance your only reason for using the <heading> style dropdown is for the purposes of building a TOC. Note however that unless your heading style is very highly specified you are still going to alter the appearance of your headings, which may or may not be desirable. This is what the current Sigil behaviour suits well as per the example from Jellby above.
Use Case 2: You are using Sigil as a book editor to cleanup a book, and the chapter headings do not already have a dedicated style. In this case the chapter headings are often intermingled with standard paragaraph text, heck sometimes even appended to the end of a paragraph block if its a really bad scan/conversion. Here is where you don't want Sigil to be "inheriting" whatever rubbish style the headings had previously underneath.
I've only ever used Sigil book view to apply headings in the second case, since other than for splitting pages it is the only time I would edit in book view. I guess I saw the heading dropdown with "Heading 1" etc similar to choosing a heading style in word - once applied, your text gets that (and *only* that) "heading style". Which used to be the case, the text would only get a <h3> or whatever tag around it.
I don't know the answer, I just find the current behaviour not useful since I am unlikely to go through a retail ePub page by page changing styles in book view to build a TOC, since regex is way faster. Anyways, hope that explains it better from my head.
