The Garmin guy is just laying out the situation of GND v. AIR. He isn’t suggesting that Garmin have removed the GND mode, AIUI (they may no longer be indicating “GND” on the LCD and/or it may no longer be user-selectable). @NCyankee may well know more.
The bigger Q is why the advice to leave them on ALT generally. It may just be to make sure that those with transponders installed fly with them ON. One less thing to forget. On a longer flight, ATC will normally ask you to squawk but those flying locally (the majority of GA) could just forget. I have sometimes forgotten to set the squawk given in a departure clearance.
Obviously every airspace authority would prefer a known-traffic environment. We can debate the % of people who have them OFF deliberately and it may be country- or community-dependent, but we did that in previous threads e.g. here where some pilots, from countries which may surprise some, make passionate arguments for the OFF position 
I forget to switch to alt from gnd, so I go to alt when I turn it on. No one objects.
Peter wrote:
The bigger Q is why the advice to leave them on ALT generally. It may just be to make sure that those with transponders installed fly with them ON. One less thing to forget. On a longer flight, ATC will normally ask you to squawk but those flying locally (the majority of GA) could just forget. I have sometimes forgotten to set the squawk given in a departure clearance.
@Peter the prior FAA policy had something to do with limits of FAA equipment in airport towers. Apparently that issue is gone and there’s no reason to switch out of ALT today. Obviously its one less thing to fool with.
Many flying long distances in the US (not just locally) have no need to ever switch our squawk from 1200 (US VFR), and if that’s the case it’s now only rarely necessary to touch the transponder. Airport towers mostly don’t assign a discrete squawk code. I do ‘ident’ once in while when talking to ATC, which serves a purpose given that few planes are Mode S equipped in the US, including mine. Discrete squawks are assigned if on FAA Flight Following but I don’t often use it.
Towers in my area all have a radar feed and I don’t detect much change in the number of calls from my local Tower asking a plane after takeoff to ‘Cycle Mode C’ (meaning “turn on your transponder, dummy” 
) so it’s not clear to me how many people have stopped switch out of ALT on the ground.
I have once forgotten to switch from GND to ALT and that was on a VFR flight plan through the mid of the beloved dutch control zones. So I just leave it on ALT. During the IR training the procedure calls for switching from STB to GND to ALT and back after landing, so I just do that then.
I learned to switch it to on/alt when receiving the squawk, and leave it there. This was in 1992. After that I have heard all kinds of stuff, probably due to garmin’s auto feature? Some rules have also changed lately. I don’t see any reason why not leave it on alt.
All the French regulations and club check-lists tell to set AlT from the beginning to the end. I was taught this way.
An IR instructor I know told me we should set them to SBY/GND on the ground because it triggers aural trafic warnings in the cockpits of the IFR boys.
So I rechecked and the regulations say the contrary.
A few times I kept SBY on the ground, and ATC caught me everytime (home base has TWR and APP with radar): I always forgot to go ALT before T/O.
So I keep it on ALT now sorry :)
LeSving wrote:
I don’t see any reason why not leave it on alt.
Airports with mode S ground movement radar are often explicit that the transponder should be in ground mode when on the ground.
Having read all the responses, I’m still bemused at one experience I had in a P28A. I recall getting a ticking off at a German international airport for turning on my transponder from Standby to Alt before lining up on the runway. An approaching airliner had – apparently – a TCAS alert as he headed for the threshold due to my transponder being on Alt and not on standby. I heard the pilot of the airliner report TCAS alert and asked whether aircraft on the ground had their transponders on; that question was asked by the tower to me (I was at the holding position for the active) – I replied with the affirmative and was told ‘In future, please only turn transponder from Standby to Alt when lining up’. The airport was EDDG….
That’s weird because they should be used to this. Also, isn’t TCAS disabled on short final, by the RADALT? Maybe an airliner pilot can comment.
Doesn’t this all depend on the transponder?
Mode S transponders have a GND mode and know what to do automatically, as the Garmin guidance that Peter quotes indicates. So you just leave them on (ALT).
Mode A/C only transponders don’t know if they’re on the ground or in the air, so the TCAS equipped aircraft on the approach may well be getting a traffic advisory against a Mode C response close to the threshold. RAs are inhibited below 1000 agl so I don’t think a target on the ground could get close enough to trigger one unless the rate of descent were excessive.
The SAFO doesn’t distinguish between transponder types, which puzzles me. Am I missing something, or are US pilots simply used to getting TAs against aircraft at the hold?
