Yesterday's SEC filings show that Bill Ackman has purchased approximately 7.5% of General Growth Properties' outstanding shares, throw in the total return swaps and Pershing's total exposure amounts to 19.9%. Before anyone gets too excited, please keep in mind that majority of Ackman's purchases took place in the 50 cent range. Also, this investment represents a tiny fraction of his overall portfolio. So why even bother with this investment?...