Users will find it annoying to switch into a mode, do one little
thing, and then explicitly switch out of that mode again. Doing so often
involves clicking on small "hit targets" in palette windows far
away from the working canvas, which takes up time and energy. Too
much "clickiness" in an interface is a known irritant.
Instead, the interface should do that which makes the user's job
easier, even if it's not easy to program, or isn't consistent
across all the palette items. Several builder programs use one-off
modes, so your users will likely be familiar with the idea.
Also, modes are tricky. A user can forget which mode is currently
active in a program, and take actions that have unintended
consequences. When using one-off modes, that's less likely; the
user can't get "stuck" in that mode.