"Pretty useless" might be too strong a phrase. The studies you linked actually show that people doing isolation exercises did see strength gains and muscle size. (bold added by me for clarity).
Whilst both groups significantly increased cardiorespiratory fitness
and maximal strength, ... no
differences were found for body composition.
Although it notes:
the improvements in MJ (Multi-joint) group were higher than for SJ
(Single Joint) in VO2max..., bench press 1 RM ..., knee extension 1 RM ... and squat 1 RM. In conclusion, when total work volume was equated,
RT programs involving MJ exercises appear to be more efficient for
improving muscle strength and maximal oxygen consumption than programs
involving SJ exercises...
There was a significant (p < 0.05) increase in MT (muscle thickness) ... and PT (peak torque) ... in both groups, but there were no between-group differences
Both groups significantly increased 1RM for elbow flexion ..., extension ..., FAC (flexed arm circumference)..., and AMC (arm muscle circumference).... Comparison between groups revealed no significant difference in any variable.
The takeaway is that you can build muscle doing solely single-joint, isolation exercises. However, there is very little carry over to multi-join exercises. Meaning, that the best thing you can do to build your squat is to squat more. There is no major benefit to adding leg presses and leg curls.
Some unilateral exercises like the lunge are also used to fix muscle imbalances in which one side of the body is stronger than the other (A different kind of imbalance that other question was asking about).