I've heard the "500-watts-into-a-4x10-is-the-absolute-minimum" thing a lot around here, and honestly it leaves me wondering what bars people are playing in or what insane drummers they're playing with. I've usually played with 200 watts into a 1x15 or 2x10 and the volume halfway up, and never had a problem being heard with a rock band in a bar. I can't visualize a band where that setup would be inadequate in a bar, unless said band were so obnoxiously ear-splitting loud it was driving customers out (and I have heard a few bands that did just that). A bigger rig might be fun, but it's hardly "necessary."
If the venue is bigger, I would assume there would be a PA and you can take a DI off your head or mic the cab. If you're playing outdoors AND have no PA support to piggyback on, then I can see saying more than a 200w 1x15 is "necessary," but only then.
What I've learned by going bigger (especially on the head) is I just get A LOT more headroom for that clarity I like.. Get the most watts you can afford (good watts, not Behringer watts) and this will afford you that headroom, that's really why, not just to be loud.