Sure, it won't "block the NSA" from discovering anything about you. But it raises the stakes.

It is much easier for the NSA to backdoor/crack/make deals with a few telecoms, collect all of the encrypted data, and then decrypt it at their leisure (via stealing RSA keys, getting them via legal channels, cracking RSA, taking advantage of compromised random numbers, or the like), than it is for them to backdoor your hardware (much larger surface of possible discovery), install keyloggers (need physical access, much more expensive to do so only makes sense to do in a targeted fashion) etc.

The whole point of this brouhaha is that the NSA appears to be hoovering up massive amounts of data, which it keeps and which even fairly junior employees at government contractors have access to and could abuse. I don't think that anyone (or most people, anyhow), object to specific, targeted wiretaps of targets that you have appropriate warrants for. It's the incredibly broad based collection of all records, without targeting, for later data mining, coupled with the first-amendment destroying demands of secrecy about discussion these practices, that have people up in arms.

The NSA has admitted that they retain encrypted data for longer than unencrypted, as they cannot as easily discard it as irrelevant. If you use a protocol with perfect forward secrecy, it substantially undermines the value of that collection.

Edit to add:

> It won't stop them from storing the encrypted communication until (for example) quantum computers make it possible to decrypt it (assuming RSA).

Actually, it will stop them from using quantum computers to crack RSA and thus be able to read past communications. That's the who point of perfect forward secrecy; even if you can crack the RSA layer (or obtain the private RSA keys), which is used for bootstrapping the authenticated encrypted channel, perfect forward secrecy generates the symmetric key in a way in which it is never actually communicated over that channel.

Perfect forward secrecy protocols, such as Diffie-Hellman key exchange, work by both sides coming up with two values, one of which is secret and one of which is passed over the channel, such that both of them can compute the same secret value by combining the private and public values. Even if the "public" values are intercepted, for instance by cracking RSA, you cannot determine the secret values, because you do not have the secret portions of the keys that each side generated.

The reason to use RSA in addition to key exchange is that Diffie-Hellman key exchange is not authenticated; someone could MITM you, doing both sides of the Diffie-Hellman protocol, and intercept your communication that way. RSA allows for signing of certificates which can be used to bootstrap the authentication process.

So, if RSA gets cracked in the future, then the NSA could at that point MITM all of your future connections. But they could not decrypt your prior conversations, which had been encrypted using a private key generated via a key-exchange protocol that provides perfect forward secrecy.

edit 2:

Hmm. After further investigation, it looks like quantum computers would be able able to break the common "perfect forward secrecy" algorithms as well. So yes, in the case of RSA being broken due to quantum computing taking off, it's likely that DH and other discrete log based perfect forward secrecy algorithms will be broken as well.

Sigh. Why can't we have any NP-complete problems that work cryptographically? While even that isn't a guarantee of security against quantum algorithms (after all, it may still be the case that P == NP), there's a fairly strong belief that P!=NP and that NP-complete is a disjoint category than quantum polynomial algorithms, and so would give us a better security margin against quantum attacks.