Lunamoth,
There are two separate, independent attributes that are involved here:
1) The survivability of memes
2) The survivability of genes
The survivability of a meme is not dependent on the survivability of a gene. Additionally, memes are spread much faster than genes meaning that their survivability has a different meaning than gene survivability.
If a gene is survivable then it's host will last long enough for it to be passed on to the next generation. If a meme is survivable then its host will last long enough for it to be passed on to the next person. For a gene, there is an inbuilt time limit which guarantees a certain amount of survivability for the host. With a meme this is not the case. Consider a meme that caused the host to commit suicide after 5 seconds but was survivable enough to be transmitted every 2 seconds. Therefore, for every dead host, the meme would be transmitted to 2 other hosts created a population boom before eventually wiping out all hosts and the meme along with it.
So it is not the case that a neutral meme will tag along and eventually drop out if they have no consequence on survival if by survival you mean our survival. This is true if you mean the meme's survival. Why are we even talking about memetics? Because meme is itself a meme with a level of survivability which is totally independent of our survivability.
Our survivability has caused us to have certain desires. Some of these desires are helpful to our survival. One of these is impure altruism. People who are impurely altruistic also desire to be purely altruistic. This creates the pure altruism meme which infects those with impure altruism as it is rational for them to accept it since they are predisposed towards it. They are then purely altruistic.