Antonio Cromartie makes Deion Sanders look like a good corner in run support. That might be harsh, but it is also difficult to debate. Cro is a bad tackler. He has very poor technique. There are many instances where he appears to purposely avoid situations where he might have to make contact. The most infamous was a Playoff game against the Jets while he played for San Diego. He had a clear shot at Shonn Greene and appeared to pull up. Greene broke a long touchdown run.
How much does this really matter? I think it is the same as run blocking for a receiver. Yes, it is a nice quality, but it is probably overblown. A cornerback's most important job is to cover wide receivers in the passing game. How many times do receivers and corners really affect the run game? It happens very infrequently. It does make a bigger impact against receivers after the catch, though.
Since we are attempting to do a full evaluation of Antonio Cromartie, though, let the record show this as a major weakness.