It means if you're scared enough by the intentional vagueness of the term, either somehow contribute to fulfill the clause or don't use the software. Perhaps that's not how it would be interpreted, IDK, I'm not a lawyer.
Is fixing a typo in the docs enough? What about bugs reports? These are useful tasks for a open source project.
You may get contribution-spam fixing inexistent errors, just to fulfill the clause.
If developer X is working in company Y and a year later X changes to company Z, who can use the software Y or Z? What happens if X dies? What happens if Y goes bankrupt and W buys all the assets?
I agree it's vague... you could say for whatever the contributor(s) decide to merge but in any event it seems easy to fulfill the cause, especially since projects need continual maintainence.
At least it leaves some leverage in the hands of the authors/contributors against flat out freeloading. Normal MIT is essentially no strings attached.
I think your understanding is correct, but I don't see how asking for a CLA is too much of a problem. It's at least more straightforward than whatever legal spaghetti you'd be getting into just to find something that fits your definition of "fair".
You may get contribution-spam fixing inexistent errors, just to fulfill the clause.
If developer X is working in company Y and a year later X changes to company Z, who can use the software Y or Z? What happens if X dies? What happens if Y goes bankrupt and W buys all the assets?