This has been demonstrated in a few competitive games this year, most notably Na'Vi vs. Darer in which Darer won through protecting Death Prophet, though in general the team fighting seemed to be largely in Na'Vi's control. In other games against Darer where Death Prophet was not picked, it was more or less completely one-sided for Na'Vi.
I know some people might think this is exaggerating, but I'd consider Death Prophet a first-round ban, in the same tier as Nature's Prophet and Leshrac. If she is protected, her team can easily force a loss just by fighting near towers and keeping Death Prophet up during her ultimate. That's it. Of course there's more execution involved but as for a general strategy that is incredibly easy and has very small risk compared to the potential reward.
I prophecise that this hero will become much more popular by the International. It will be interesting to see if any real hard counters can be found by then too.