I think it depends on the bonus conditions if you have to bet the bonus money 40 times or more then the chances of winning anything with the bonus money are very small.
All the bonuses I have seen have similar conditions to this one you mentioned. So I think there is no point in taking bonus as it's almost impossible to wage our balance 40 times and quit the game with profit.
On the other hand, that condition is what makes the bonus possible to be offered by the casino, as they know most gamblers will fail, then they can assure the big prizes for few winners.