Another thing to remember: browsers don't fire up unlimited requests, usually you have 6-8 pipes to get your assets through. Even simple if-modified-since will

Another thing to remember: browsers don't fire up unlimited requests, usually you have 6-8 pipes to get your assets through. Even simple if-modified-since will still take space in them, while "eternal caching" will leave them free, i.e. no request will be made.