@theducks - don't confuse "jacket" functionality with "cover" functionality. Removing a jacket does indeed just remove the jacket.html or similar page without touching the css.
@PatNY - most definitely the plugin does not do a whole conversion, its entire purpose in life is to avoid doing them for the negatives that can occur. However it does use parts of calibre code that also gets used by conversions - such as smarten punctuation for instance. In the case of inserting/replacing a cover it applies some similar logic to that done when doing a conversion, included in which is making sure to use the same calibre default settings for whether to use svg images and proportioning.
As for preferring stretched or non-stretched covers, there are people on both sides of the debate and it isn't really one that should be had on this thread. The plugin supports whatever setting you prefer and have specified in your calibre defaults.