"The Mars Direct plan total mass is 53.8 tonnes. Of that 10.4 is consumables. The consumables during transit would be actually be about two thirds because the plan also makes use of in-situ resources for O2 and water. So roughly 7 tonnes of a 53.8 tonne mission is required due to the long trip time to Mars."
Hmm, if you are talking about total weight per person (so, times the above fiqure by 4 [for minimin size Mars crew]) then you are somewhere in the ballpark.
http://perso.wanadoo.fr/salotti/lifesupport3.pdf

"That's less than 15% of the total."
What total?
Gross mass at lift off? Spacecraft mass which is in LEO? Spacecraft on a Mars trajectory? (Or perhaps, dare I say, from L1?:)

"Since the delta v to land on Mars is actually smaller with aerobraking than landing on the Moon, the energy requirements to land the same amount of mass at either destination would be close enough that one launch vehicle design could work for either destination and the same amount of mass."

No. The size of mass of spacecraft would only be similar if you had similar durations of the Mars and Moon missions.

"You could probably cut corners and get one to work for the moon that would be marginally too small for Mars delivery. Pretty stupid planning if you chose that approach."

Keeping It Simple, Stupid is a good engineering approach. "Stupid planning" would probably better than "overly smart".
In other words, using the Hvy boosters we currently have to go to the Moon, makes a lot of sense. Compared to having NASA building a new Saturn type booster- "so that it might be easier" going to the Moon and Mars. Or if you are designing a lunar lander you would design it for landing crew on the Moon, but it makes a lot of sense to design it so it could have other uses (with slight modifications) so it could be used for Mars, NEOs, even large robotic missions. These modification would also have added benefit of allowing more flexibility in regards to solely a lunar mission- for example the lunar lander could be modified so a similar vehicle could be used on Earth return to LEO or having larger payload capacity to lunar surface. There are, of course, also aspects, which would be part of the detail design. So, by having engineers "allowing" for future Mars missions makes a lot of sense, but this shouldn't considered a strict requirement.