Allegedly 'practical' arguments assert that we should enact the policy in question because it will have some consequence. For instance, one 'practical' argument in favour of using extraordinary rendition holds that it will increase public safety by enabling us to catch more terrorists. But that means nothing unless we accept a prior principle that the consequence in question is desirable, i.e. that we should enact it.
Calling such an argument 'practical' is mostly a way of shirking the burden of defending such a principle. It is a rhetorical sleight of hand - a way of saying, "Look, this principle is just so obvious that I shouldn't have to explain it, or weigh it up against the principles that the other side has advanced."
The hidden principle could be weak or strong. If it is weak, e.g. "Other things being equal, we should do that which enables us to catch more terrorists.", then it's not clear that the premises of the argument entail its conclusion. The whole point of the other side's case is that other things are not equal; for instance, that extraordinary rendition is brutal, inhumane, and damages the autonomy of ordinary innocent people. If the hidden principle is strong, e.g. "We should always so act as to maximise the number of terrorists captured.", then it's not clear that the principle is so obvious as to require no defence.
