One being balancing traffic equally across multiple caches, and the other being how to handle extremely large and extremely popular content while avoiding duplication of data. L4 handles the balancing beautifully. L7 switching is nice for a lot of things, and is a wonderful technology, but we don't need the compexity of it to ensure that there is little to no replication of data across caches. An L4 switch hashes the IP to choose which cache data goes to. I'm not terribly familiar with the proprietary L4 switch offerings, but the new LVS hash selection algorithm does not present problems wrt persistence, as every request to the same IP will always go to the same cache, assuming it is still in the pool of viable caches.
Persistence is automatic.
It's like magic. Think of the power of a carrier class cache. This is where L4 switching works beautifully. Redundant array of inexpensive caches. This is a pretty nifty idea.