As mentioned above, I've had non-OFM planets use fleets in the defence. I'm only using 1.1b1, so perhaps this has been fixed.
Frankly, I think the 'one ship per move point' thing has to go. Fine, if you've got plenty of defenders, but empty hulls? Transports? Without an OFM, they should be destroyed last, and all at once. I figure attackers would go for the warships first, and the defender could screen the weaker ships, but there's no way five empty ships can both run away - so as not to be killed - AND stop the enemy ships landing at the same time. Really, the little 'shield' should mean that there's something in orbit that can stop transports landing, and empty hulls or freighters etc can't do that.