Long Expires/max-age dates, why its suggested?

[pmeenan]

The long expires are recommended explicitly because it is recommended that you change the file names every time the files change.

While an if-modified check won't use bandwidth, the round trip to ask the server is really expensive for performance (and often as long as downloading the resource itself) so there is a big performance loss in that case.

If you can't rename the images when they change there isn't much you can do, it's just something you should be aware of.