MtGox handles ~80% of all exchange trade. When people buy bitcoins on OTC or in person or so forth that doesn't show up as exchange trade. When people buy bitcoins by renting equipment from miners that doesn't show up as exchange trade. These two markets are each larger than the exchange trade, so MtGox handles maybe 20 to 35% of "traded bitcoins", and this mostly because trading bitcoins for actual items is rare and low ticket yet. Somebody was offering to sell a house for bitcoins, and so this third market is probably going to baloon larger than MtGox's in the near future.
In short no, MtGox is not nearly as important as it makes itself out to be, nor does it have much of a future at the current rate of misbehaviour.