some users undoubtedly do behave like you're describing. Many others just go into the forums they want and look topic-by-topic. People who may find the site by a search engine might only see the front page, or a specific topic. Some people might only come and use the messenger to talk to their friends in there. Search engine spiders, or other crawlers, might hit any one of the dozens of available links on the page. In most cases, it's the board index that is the most loaded page, by a wide margin, that's the one that needs to be resource friendly.
No one is saying that this is a bad idea, I'm sure lots of people would like to see this as an available hook, it's just that it's not something IPS can unleash on every board, it will cause resource issues on the biggest boards that IPS then has to deal with.