I thought its easier to delete the whole tag. DEL deletes the next character and when i'm "in" a tag i thought something like CTRG+DEL or ALT+DEL can delete the whole tag.
Code:
<sp|an id=bla">a long text</span>
Lets say the | is the cursor position. Calibre editor (and sigil) highlight <span> and </span>. Now it would be cool it could delete both by CTRG+DEL or something else. Going to the menu is not faster as deleting the two parts of the tag manually.