I'd comment that this is not limited to a summer/full plane phenomenon. I mostly travel in paid F/J (I'm hostage to US/AA in PHL), but I'm moving nearly all of my Int'l travel to BA. BA isn't Singapore, but it's light years better then US/AA (Even with the US PBD).

It is no cheaper to fly a domestic airline internationally, I do lose MM miles, but the difference in service is worth it to me. You would think at least on the international flights the domestic airlines would try to ensure good service, due to profitability.