It must cause problem when someone want to optimise Haskell program.
If a programmer find that program is slow, then he profile and try to optimise it.H e find bottleneck long computing function, and he use thread to fix performance problem (Of course, bottleneck isn't memory efficiency in this case).
But now he can't profile his proram again. forkOS cause error without -threaded option.par can work without -threaded option, but profiler doesn't report correct result.
That's too bad.