Connecting a lower voltage load to a series of higher voltage batteries in halfway?
The unbalanced load will unbalance your batteries. This may or may not be a concern depending on the details of your loads, batteries, charger and expectations. 

If the 48v load is less than 1% of your 72v load, then you can probably ignore the imbalance and let the charger rebalance the batteries. 

If the 48v load is significant, then you'll have to handle the fact that some batteries will run out before the others, and wear out before the others. The charger may damage the single battery providing the 72v, and may not fully charge the other two.

If one load is much less than the other, it will always be better to use a small converter to power it from the full battery. If they're similar sizes, then you have to think what's really important, cost, run time, reliability etc.