I was always under the impression that while the saucer could be separated, it still was never intended to make a planetary landing.
I think the idea of having the ship make any kind of landing fluctuated throughout the development of the show, and was only added back on as a latter day revision. Initially, Jefferies' development sketches alluded to the
hero-ship being able to land, much as the C-57D had in
Forbidden Planet. From what I remember reading (In Whitfield's
Making of Star Trek I think) this was considered too expensive to film on a regular basis for weekly network episodic television. The workaround, the creation of the transporters, resolved how to cheaply have the crew leave their vessel to go on adventures planet-side without killing the effects budget. It seems likely to me (although Gary Kerr, Mark Gagen or Shaw would definitely be more up on this, or have the facts at their disposal) that Gene Roddenberry asked Matt Jefferies to add the triangle features to the construction blues for Datin to allude to the saucer having that ability. Inferences, call outs and dialogue from the "The Apple" and other episodes seem to have swayed Andy Probert's thinking in that regard as indicated
here- but he
never states that Gene or Matt Jefferies made
any definitive statement, one way or the other. So, while the TOS
Enterprise doesn't definitively have that feature, it certainly does
post TMP. So again, YMMV.
My thoughts: The round, cylindrical portion on the ends on the triangle features do looks like hinges, but I prefer to imagine them to drop "open" as the ship's reverse impulse thrusters. The triangle shape is a coating to protect the hull from the exhaust. Additionally, these vectoring thrusters can be directed downward to help blast the ship out of orbit in emergencies. YMMV

.
You make a Interesting point - it's a similar idea to what's in the SHIPS OF THE STARFLEET Volume II Akyazi class perimeter action ships on the bottom of the primary hull of the Akyazi ships with Guenther's Emergency Impulse Engines. As for the design of the triangles, I would consider them to almost act as aero-braking flaps, but since I've never taken a any courses on modeling in any kind aerospace/aerodynamic aspect, I can't speak to whether that would work in regards to the TOS
Enterprise primary hull. Still, not a bad idea.
You just reminded me of my favourite unasked/unanswered question for Richard Taylor and/or Andy Probert: Did someone consider providing alternate optional photon torpedo and impulse engine assemblies for the Refit Enterprise during the redesign process and are these features a result? On the Refit Enterprise the unmarked thrusters ports bear a strong resemblance to torpedo doors on a submarine. The existing photorp launcher assembly bears a marked similarity in structure and design to the impulse engine assembly. Was it perhaps an idea for the photorp launcher assembly to function as the forward impulse engines in an alternate effects configuration? Probably not, but I made a example of what I'm talking about below just for the fun of it.