Shape builder, despite the rounding bug that is embedded in its core (https://illustrator.uservoice.com/forums/601447-illustrator-bugs/suggestions/37424191-expanding-a-live-paint-object-changes-dimensions#comments) is still a very useful tool many users are very fond of.
Specifically, it can be used to bring pathfinder options to open paths — you can delete parts of lines that are covered with another shape, effectively subtracting segments from them (rather than closed shapes), by Alt+dragging over this shape (see the GIF attached).
But this nice trick won’t do the job if lines don’t cross the entire shape. That is when a line ends inside of a shape, the tool doesn’t trim it.

I tried to find a reason why, and could not.
So please fix it, along with the rounding bug above.

And I can’t but mention that there’s a brilliant script that uses the LivePaint tech to give us the ability to subtract/intersect shapes from/with lines without manual efforts like these, something that the Pathfinder should be able to do from the box (and we have a request for this too: https://illustrator.uservoice.com/forums/333657-illustrator-feature-requests/suggestions/34003474-pathfinder-to-work-with-open-paths) — the script is called SplitPath, by Sergey Osokin, https://www.youtube.com/watch?v=1_vUUFkTwxk