Not entirely, useless, I grant, as they seem to mean something to a lot of people, so they have rhetorical uses of some kind. (I think this is the late-Wittgenstein defense.)
And of course they’re attached to the USA’s two main political parties, so they’re used as substitute words. (I think this is the tautological defense.)
But, seriously: Conservatives are not, in fact, fiscally responsible. They’re much worse than Liberals on that score. For example, compare “conservative” Governor Sam Brownback’s fiscally irresponsible economic leadership to “liberal” Governor Jerry Brown’s–in the same years. One injected the hallucinogenic drug, “trickle-down economics” into the veins of Kansas, which now has to go to rehab. The other dealt with deficit spending well and helped California find its way out of the post-2008 doldrums, chiefly by following mainstream economic “theory” and forcing the legislators to compromise. In the U.S. Congress, “Conservatives” want to address deficit spending by cutting taxes to very wealthy people, gutting health-care, but raising the defense-budget. As a famous economist once said, “What a mess.” The labels don’t really tell us anything, and more than that, they confuse things; for one is more likely to get prudent (“conservative” in that sense) economics from Democrats these days–which isn’t saying much as their only competition is Brownback and other nuts.
Conservatives like playing nice with dictators. That makes them authoritarians if not fascist-leaning. Liberals do the same thing, with the exception of Assad and Putin. The labels are useless.
“Defending” the Constitution? It’s always a tie, as the defense depends on the issues. Conservatives gutted the Voting Rights Acts and enabled lots of Jim Crow voter-suppression. Does that sound like defending the Constitution to you? They blindly support excessive police force: again, more fascist than Constitutionalist. Liberals want to interpret the Second Amendment’s whole sentence, not just the second part, so as to allow gun control. Is that defending the Constitution? Hard to say.
Foreign policy? Okay, Trump’s worse than a disaster, but that’s not owing chiefly to his being labeled a “Conservative” (for the moment). Bush and Cheney were more war-mongerish than Obama, but Obama maintained the wars and, like them, used drones. They supported Israel exactly the same, propaganda to the contrary notwithstanding. With regard to the defense budget, Democrats give the Pentagon whatever it wants, Republicans do to, but the GOP goes further and gives them stuff they don’t want. Both parities are fully pro-military. It’s not a close call.
You say liberals are more sympathetic to Civil Rights than conservatives? For the sake of argument, let’s say yes. But why is that not a “conservative” thing–conserving civil rights? That’s right: because the GOP decided @50 years ago to become Dixiecrats with regard to race. “Conservative” doesn’t necessarily mean “White Supremacist,” however. Anyway, liberals talk a good game, howling at the misbehavior of stock brokers and bankers who leveled the economy. But I didn’t see Obama and Holder sending any of them to jail.
All of which is to ask Political Scientists to come up with more accurate, less ritualistic labels for politicians that describe. I’d also go for a one-month ban on broadcasters’ and journalists’ use of “Conservative” and “Liberal,” just to see what new words they might bring in.