The money they could potentially earn from fees is much more than the interest they could earn holding it for a few months.  Not to mention, they would be illegally exposing their customer's funds to risk of loss.  It is much easier for me to believe that they simply are having trouble finding banking partners that aren't throwing every possible reporting form they can find at MtGox for every transaction.  Nobody knows exactly what regulations apply to bitcoin, and banks are risk averse when it comes to regulations. especially for something they view as small potatoes (compared to their other businesses).  Because of this uncertainty, they make MtGox report everything, and they set very strict limits on how much volume they can process and how fast they can grow their volume.