I think we should allow these links, because I don't think links to file-sharing sites that don't require account creation actually pose a risk greater than other kinds of download links.
There is no reason to think that links to sites that require creating an account (like Dropbox) are safer. There's nothing stopping a bad guy from creating a free account. This applies not just to Dropbox-style sites, but also to SourceForge, GitHub, Launchpad, and the like. Anyone can make a Launchpad account with a fake identity, sign the code of conduct under false pretenses, and roll out a PPA to infect users with malware. The PPA could even be named similarly to an existing PPA, to fool users. (To the best of my knowledge, we haven't seen this yet, but that's only because Ubuntu still occupies a pretty small market share--there probably aren't enough users yet--that is, potential victims--to make it worth the time, effort, and risk.)
Such a miscreant could even create and use a paid account on an online service. Rather than going into deeply instructional detail about different ways to do this, I'll just point out the simplest: A criminal can con someone into trusting them, give this initial victim (real) money, and get the victim to create the account in the victim's name, for the criminal to use.
If practically speaking there are specific websites that are shown to be used on AskUbuntu, other Ubuntu support resources, or other StackExchange sites to fool people into downloading dangerous files, then that would probably warrant an automatic warning message cautioning users, and maybe warrant a policy of forbidding links to those sites. But fundamentally, anonymous file sharing sites are only needed by those of us who aren't criminals taking over people's computers. If you can take over people's computers, you can run your own download servers. You can even register (free or not) DNS for them. And you can always enjoy anonymity online with a fake identity.
With that said, providing any kind of direct download link for software (except apt: links for installing packages from the official repositories) should be strongly discouraged except in the rare cases where it is demonstrably necessary, and in those cases, users should always be cautioned about the risks.
I think we should
start continue disclosing the potential risks of PPA's, too (in every post that suggests installing from a PPA, or automatically for posts containing PPA syntax). [EDIT: Jorge Castro has reminded me that we're essentially already supposed to be doing this.] This would not be to keep people from using them. Rather, assuming Ubuntu continues growing in user base, eventually PPA's and other non-official software sources are going to start being exploited to fool users into installing malware. It would be good for Ubuntu users to be prepared for this, before it starts happening.
In conclusion, making a distinction between safe and unsafe downloads is only a good thing if the safe downloads are substantially safer than the unsafe ones. What it means for a site to allow anonymity is that it lets rule-abiding users go without disclosing their identity, because rule-breaking users can and will always be willing to break the rules and create a fake identity that looks real. Singling out and prohibiting linking to download sites allowing anonymity would only give us false comfort, and make us all the less prepared to deal with threats effectively.