Almost forgot another downside of onscreen double-taps - once you use it in an area, just can't use it for fast single-clicking anymore.
Once you get the first click, you would have need to introduce a slight delay to decide if the input was a single click or doubleclick. This breaks fast paging and needlessly delays GUI response for single-clicks.

I hit this issue in Mieru - the whole screen is divided in half and each half is used for forward/backward paging. Paging is activated immediately once the user lifts the finger & every click turns a single page forward/backward.

If I added double-tap support, this would introduce unnecessary paging delay & users won't be able to quickly page through many pages.

In the end it was solved with a special edge paging mode, where the screen middle can be used to perform different actions by single & double-clicking the screen middle. Users can still page quickly & the middle click actions don't need to be perfomed in rapid succession (fitting mode switching) so the delay is not a problem in.