Any chain that you can carry on a bike can be dealt with by bolt cutters or a hacksaw.
Security devices have a number of purposes:
1) They effectively repel impulse thieves. That is thieves who just want to grab your bike and run. If your bike is chained and locked, they have to be able to deal with the chain and lock.
2) They slow down even prepared thieves.
3) They might deflect thieves. If there are a dozen bikes of equal value in the same location and one is locked and the others are not, there is a good chance that thieves will take the other bikes rather then fool with the secured one.
That being said, any bike left outside unattended for long periods of time is vulnerable to theft. Of course, a really cheap bike with a big old lock on it should be reasonably secure. It is unlikely that anyone would go to the trouble to steal a cheap bike.