It probably wouldn't help too much on the front page, since that'll update constantly and therefore just be a waste of overhead.
I disagree that the only reason not to do it is if the data won't fit in RAM. Going to a multi-server setup has many advantages including redundancy for situations such as these, of course, at the cost of increased administration.
Any reasonably large site will (relatively) quickly exhaust RAM, especially if the same server is handling the requests.
Plus, traditionally there are large sets of data that needs to be searchable, hence the desirability of putting it in a database.
So instead of actually having things in memory, people now use something like memcache to reasonably simulate that for performance.