The only way you are getting a smooth (tear and judder-free) video on your display is if you run the game with v-sync on and never drop frames. V-sync introduces very noticeable input lag no matter the situation. Multiplayer FPS games are basically unplayable with v-sync, for example.
Without g-sync/freesync, your only options are to deal with this input lag or deal with tearing. So it's far from useless if you ask me and worth the extra cost just for this reason.
144hz works the same way as 60hz in that you still need to use v-sync for smooth video. Thanks to the higher refresh-rate, however, playing without v-sync isn't as painful as on a 60hz monitor due to the tears being less noticeable.
Consider this, though. Getting a constant 144 FPS in games is pretty rare in my experience. Even triple Titan X's trying to run maxed out modern games can rarely do the job. Many of these games are definitely going to be running at less than 144 FPS. You can sync to half the refresh rate (72 FPS) as an alternative but then you're just wasting the opportunity for smooth motion on a higher refresh rate monitor. On a more standard rig, the variable 90-110 FPS you might get in a specific game will be a far more pleasant experience on a g-sync monitor than on a regular one as you either will get tearing or have to deal with a locked 72 FPS. There's also flexibility in what you may want to do. On the g-sync monitor you can lock this same game to 90 FPS without incurring any input lag due to v-sync and get very smooth video. On the regular monitor you'd have to add a 90hz profile just for this game and use laggy v-sync.
So no, it's far from useless both in your hypothetical situation and in actual practice.