One of the funniest (or annoying) things about playing Warhammer 40k is the surprising amount of players who, for whatever reason, have not read their codex or their rulebooks. The Tau Landing Gear is the prime indicator of someone who has not read the rulebook or the codex. Ever been in that situation when your Hammerhead gets immobilized by a lucky shot by a Lascannon?
Your opponents gets all excited and says "its destroyed right?!"
"No," you say, "it's just immobilized."
"Oh because it has landing gear?"
"No because it didn't move flat-out, silly. What's landing gear got to do with it?"
You see, this situation happens to me all the time and its an indicator that your opponent doesn't know the skimmer rules, nor does he know the landing gear rules.
A skimmer is only destroyed by an immobilized result when it is either in a vehicles squadron (as per the squadron rules) or if it has moved flat out (more than 12 inches). As a Hammerhead or Devilfish are not fast skimmers and cannot move flat out, nor are they in squadrons, an immobilized result is only ever an immobilized result.
Landing Gear has nothing to do with immobilized results whatsoever. It simply allows a vehicle to land in the movement phase. While it has landed, it does not count as a skimmer. I have never used this ability. I could see it being used to create a LOS blocker. A Devilfish lands, removes its flying base and a squad of Battlesuits hides behind and jumps out to shoot and then jumps back. I've honestly never done so, as I like to keep moving but it is just one trick in the bag you can use.