MySQL does already automatically cache queries internally. However, the scale of that caching is limited, in part because it's difficult to scale MySQL horizontally in the same way as Memcached or other distributed caching systems.
Adding read slaves is a great on many levels. Adding slaves, however, is not without overhead and stale data/programming complexity exists there too.
The caching that MySQL does effectively useless unless all of your queries deserve equal priority (times their individual footprint) for memory. That's almost never the case. I don't want relatively low-impact-rare-reads to take up space that I'd prefer to use for high-impact-common-reads, for instance.
It's nice that MySQL caches queries, but it doesn't solve the same problem that an application-level cache does.
Adding read slaves is a great on many levels. Adding slaves, however, is not without overhead and stale data/programming complexity exists there too.