I don't know much about Tahoe-LAFS, but if there's no built-in incentive to provide storage, I'm guessing there's also no built in verification that you're actually storing what you say you are.
In Tahoe-LAFS there's a verify-capability associated with each file that lets you do an integrity check over the ciphertext and erasure coded shares for that file. A server can only lie about storing a file until another node attempts to verify it, and this can easily be scheduled to happen periodically.
I suppose the verifying node will itself need a copy of the file (or the part of the file it's verifying)? Then either the owner of the file will need his own node to store the files and verify (which somewhat defeats the purpose), or he will need to rely on other nodes to quiz each other. What incentive do they have for this, and in case of a conflict, who should be believed? What are the penalties for failure to verify?
Maybe there are already solutions for all of this, and if so it's great. But again, a fully monetized platform is a league of its own in the requirements for a comprehensive verification protocol. As I understand Tahoe-LAFS is not currently a fully monetized platform, so I have my doubts that such a verification protocol has been developed.