In most cases it probably would, but I wonder if you could
make javascript mine with gpu (using WebGL for instance).
Then if you'd have enough visitors, this scheme just might beat
ad networks in profitablility.
Of course, you'd have to be careful to make clear to visitors
that you're taking control of their resources while they're on the page,
and provide some content that is valuable for them in exchange
for that.. Then it would be just like running a mining pool except you get to
keep the coins and the workers are paid out with content. Hey,
let's mine some porn today!
interesting thought..