Although the exact trajectory of a rocket is not known before launch, could SpaceX position its landing barge even further down range of the launch pad and eliminate the need for a boostback burn during barge landings. This could allow heavy missions such as Echostar 23 to complete a very low margin barge landing.
One problem is that towing speed of the ASDS is pretty slow. Around 5-8 mph. This means that to get 500 miles off shore, you need almost 100 hours of towing to get there, and 100 hours to get back.
That is a lot of time to pay staff, and ship usage. Also it reduces turnaround time if you need more time to get back and forth.
Of course, the longer at sea, the more damage the barge takes. On the way back, a short trip with a 140 foot tall structure on top of a bouncing barge is better than a longer trip. The chances of bad seas damaging something or just even big waves causing too much stress on the stage increases.
Clearly there is a tradeoff on payload, recovering at all vs expending, and so on.
SpaceX has been experimenting as discussed with many of these variables and will optimize to what makes sense, if you had all their data.
The falcon 9 has 3 return modes, depending on the launch profile.
The first involves barges and has 2 burns. The reentry burn to prevent burn up in the atmosphere and the landing burn, there is no boost back burn as the rocket stage continues on its parabolic trajectory guided by the grid fins.
The second involves 3 burns and landing on land. This adds the boost back burn which reverses the direction of travel followed by the reentry burn and the landing burn.
The third also involves 3 burns but lands on a barge. This uses a boost back burn to limit how far down range the rocket travels followed by the reentry burn and the landing burn.
For heavy launches the first method could be used to allow more fuel to be used in the launch.
So in effect you are exactly right, a barge landing can be more efficient because there may not need to be a boost back burn.
But the above does not take into account the points raised by @geoffc, that a rocket at sea is at higher risk for damage after landing than a rocket on land. We can assume Spacex has considered the risks because they continue to use the less fuel efficient boost back to launch zone method when it is possible.