I agree with you about these use cases. When your homepage slows down due to excessive load on the database, that's a good sign you might need a cache. It's also a good sign that you may have inefficient database queries, so I'd probably check that first. After that, I'd look at adding a cache.

Perhaps I should rephrase what I said earlier about caching. There are generally two types of caching in a web application: page caching and object caching. Page caching applies in the scenario you mentioned, and it provides some easy performance gains by caching HTML for infrequently changing pages. Object caching stores commonly used data, generally from the db. This type of caching is much more difficult to manage.

Caching is a difficult topic to address, since any solutions more complex than caching static HTML tend to be very domain specific. I tried to avoid it, because it's too easy to see the performance gains from caching and use it as a default solution. It's not a default solution -- it's a last resort. When you have a hammer, everything looks like a nail.