Well, in most exercises—Squat, Bench Press...
There really isn't an issue with fully locking out.
If you stop a quarter inch short of full-lockout you'll know it and it will make the exercise dramatically Harder.
But with a few exercises...
Do you do Leg Extensions?
Given a machine with the proper holes bored into the back, it can be conclusively Proven that no one ever comes to Full-Lockout on the Leg Extension after the first 3 or 4 reps—unless they're using a "Spotter Pin".
Yeah your Legs may feel straight. They may look straight. They ain't fully straight. If they were, then the weight stack would be touching the pin in the back.
You can't expect any benefit whatsoever if you're cheating (by stopping a quarter inch short of full extension—or by using Momentum.)
So most folks who do Leg Extensions are exposing their knees to Shear Forces and gaining absolutely nothing in return.
Even though they may think Leg Extensions are doing something for them...
Yeah, just doing the last few degrees does sound like Isotonic/Isometrics doesn't it?
I guess there may be some benefit to the first three or four Leg Extensions that do fully lock out...
But then how to know when you're no longer locking out 100% and might as well quit?
But I was mainly asking if anyone knew of any other Exercises where there was a Strong Tendency Not to fully Lock Out—and that would benefit from some sort of objective measure.