That exposes the entire Bitcoin system to the artistry of every pump-and-dump team that operates at MtGox.
It's great for the professional speculators, but it's not good for the system as a whole.
However, given the usual human herd mentality this idiotic situation persists.
Serves everyone right who continue to perpetuate the trading centralization around MtGox. So, party on!
Let's assume for a moment no centralization and no market manipulation. Just people trading and speculating, looking to profit. Would you expect something as new and fast-growing as Bitcoin to have a stable exchange rate?