To get large blocks incorporated into the blockchain, you will have to pay transaction fees based on their size. It could be an interesting exercise to compare this with storage costs for the same level of long-term redundancy elsewhere---but then again, what is the level of redundancy? If storage in the blockchain does become common, it is obvious that soon most people will (have to) stop storing the entire blockchain locally, defeating the purpose of having a highly accessible and highly redundant storage (for your website example or maybe for backup).
In you wikileaks example, I think this might pretty much defeat its purpose. Although many people may be interested in many of their leaks, surely only a small fraction of bitcoin users will be that interested that they'd be happy to store it without reimbursement for their wasted storage space. And at a reasonable compensation, the person or organization leaking it must be so rich, you'd wonder if it was cheaper to finance political campaigns that can change the system to allow risk-free whistleblowing at lower expense.
Also note that wikileaks is not in the business of releasing most of their information. I believe what they have distributed in bulk is a very strongly encrypted version of everything, and very selective, carefully edited small bits. The encrypted bulk would not give the benefits you imagine (allowing anyone to browse it)---and luckily also none of the risks, I might add (just imagine any of what the media has actually redacted out of their stories included something actually dangerous, like, say, the key with which some madman could trigger WW III). And it is not needed: As I understand it, plenty of people have downloaded and spread their encrypted bulk data for free and out of their own volition. I suspect if you wanted to get it, you might still find it easy to find online. Just what you might hope from getting out of browsing an encrypted monster-sized file is a bit beyond me; it seems to me the only benefit is knowing to have helped slightly with wikileak's self-insurance policy.
