Is it bad practise to answer questions that do not comply with the sites rules?
It is sub optimal, answering off topic, crappy questions implicitly encourages them.
There are procedures/policies/processes in place (down vote/vote to close/automatic deletion) that help improve the quality of Server Fault.
If a less than topical/bad question gets answered and the answers get up voted sufficiently then the automagic deletion routines fail to delete it. It gets indexed by google and this draws in more people with the same issue(s) - this is an example of broken windows theory in action.
This can clearly be seen with cpanel et al. We ruthlessly close *panel questions as off topic yet they keep coming. People even complain that SF is honey-potting cPanel via google, this couldn't be further from the truth.