It seems that they initially planned to use the landing legs as air brakes.
Elon Musk stated that:
If you then do some interesting things, like look at our landing gear,
they're essentially like giant body flaps, so the drag - when we
deploy the landing gear, the drag massively increases, so we have dual
use of the landing gear as giant body flaps and as landing gear. That
actually cuts the terminal velocity in half and therefore the fuel -
the propellant we need to stop the vehicle in half, and actually it's
quite an efficient method of landing precisely.
This maybe just an earlier idea which didn't materialized. Looking at the earlier reusability video, you see that legs are partially deployed as air brakes. But with the latest video, legs are no longer used in that manner so it is likely they gave up on the idea. My speculation is that they indeed intended to use landing legs for attitude control during the unpowered decent, but didn't work well during the initial test landings out in the ocean and switched to the idea of grid fin higher up on the body.
Also came a cross a picture that they were wind tunnel testing the deployed legs at one point.
To give a little more context, I like to add that the above comment was made on 10/24/2014, after two landing test flight had been conducted with the landing legs, flight 9 and 10 on 2/18/2014 and 7/14/2014. But also, on 6/17/2014, they already tested the steering fins for the first time on F9R test flight and only 3 months after the comment, flight 14 flew with grid fins on 1/10/15. So it is likely that by the time he made the comment, it was already decided that the air braking with legs is no go.
There was also a twitter exchange on this subject on 11/22/14 and he stated:
Using legs as air brakes to drop terminal velocity in half requires
slight redesign & more data. Maybe flight 21.
SpaceX news article on 12/21/15 before the Orbcomm-OG2 launch has a conflicting information regarding grid fins.
The first attempt to touch down softly on water failed, as we tried to
control the rocket with small attitude thrusters alone. While it works
well for a smooth, blunt body shape like Dragon, that turns out to be
a hopeless proposition for something the shape of a rocket booster.
Falcon spun out of control and smashed into the water at high speed.
We then added four grid fins in an X-wing configuration to give us the
necessary three axis control under high dynamic atmospheric pressure,
which peaks at 1.5 tons per square foot.
This solved the control problem and we were able to do two successful
soft landings in the water. Max altitude of the rocket stage was 210
km, which doesn't matter a lot, and max transfer kinetic energy was
The first attempt must be referring to flight 6, and two successful
soft landings in the water can only be referring to flight 9 and 10. But the grid fin were not flown until flight 14, so the success of flight 9 and 10 cannot be attributed to the grid fin as mentioned in this article. Well, obviously, that article was written in a hurry as stated at the bottom:
T-zero in 15 minutes, so have to sign off. Apologies for any typos in