for web-sized-pics and (instagram like) filter-effects (even effects stacked upon each other) they are cheap enough, but yeah, that question will only be solved with real world performance tests.
from my point of view: most of the canvas (effect) libs smell like premature optimization to me. could be wrong, though.
The context is within an thread on Animation. Yes, of course it's cheap enough for an effect you are just generating a still of, but it is a measurably slow approach for anything that needs animation.
And couldn't you create the kind of library you wanted by simply making a wrapper that called did something like
function X(canvas, ...) { var C = canvas.clone(); oldX(C, ...); return canvas }
You can't really do the opposite and get the performance advantages.
from my point of view: most of the canvas (effect) libs smell like premature optimization to me. could be wrong, though.