Is AR defense even worth it?

Well, the hope is that you might take one or two down at the very least.
Any kill your def team makes is 20 less lift lost

Trying to get perfect defenses is a pointless process for f2p people, AR is made for whales, and this is shown by it being mathematically IMPOSSIBLE for a f2p player to get into tier 21 without blessing and mythic abuse.

Your best bet is to just try and make beating your defense team unscathed as hard as possible, or just making nearly impossible to break both of your pots everyday, theres a method of doing that involving the obstruct seal, which requires a unit with pass to beat, and no one uses pass.

As someone with a core of Roy, Kliff, Gordin, Lilina, and Azura. You can. I have gotten defense wins non-stop and all I did was do like a 10 minute or less setup with my characters I used in arena.

I don't consider myself especially good at defense - I do manage to trick a unit out of people here and there, but I haven't figured out how to do any better. Players in T18+ are very strong, often good at finding tiny little cracks or some of the few things they can do to poke through, and to make it even harder, a lot of them have lv 3+1 offensive fortress. I do think it's worth trying to get a kill or two. It does add up over 5+ scored defenses.

Also, now that light blessings are getting handed out for free, we're going to start seeing more players in T17-20 who are a bit incompetent and who depend on blessings to climb tiers that strong players will climb without blessing bonuses, so if you have a really clever defense setup you might actually take 3+ units off them.