Ah, for me it depends on the gym. I'm not adding a defender if I know that gym is in a certain team's territory and will just tear it down later (like you want to do, apparently). It's not very potion efficient and frankly, it can get pointless except for a daily coin bonus. I don't know if I'd fault anyone for choosing not to.
You know what grinds my gears!!!
When idiots defeat a gym and leave it empty. They do not add their defender Pokémon. Sometimes, I look at the blank gym waiting to see if an Instinct player or Mystic player to add their Pokémon so I can take it down. If a Valor player adds their best defender I will most likely prestige the gym so I can add mines. This really grinds my gears.
Answers
Leaving it gray also lets a lot of low level players enjoy gyms...When we first started playing, we loved finding gray gyms so we can get 10 coins for once...
I see little kids run to gyms to get in when it's gray when it's always normally occupied by some 2500+ Blissey that they have no chance of taking out.
Love team gray, they are friends to newbies.
The only thing with that is it does irritate people, so hope they don't find out who you are. Someone did that to me, left my Snorlax...I actually wanted him back...found out who did it and took down a couple of their gyms. My gym went back up to level 10 and I never got my Snorlax back, nor is he still on the top.
Doesn't work, prestigeing it up to 2000 creates a free slot and adding a mon creates another one because adding a mon gives 2000 prestige. A guy recently tried this with me because after he spent almost an hour to bring down one of the numerous blue level 10 gyms in town I showed up in a tee with a big blue Team Mystic logo and gave him a wink, he went stoneface and left super pissed leaving one mon in the gym thinking this annoys me :)
How does this make someone an idiot? I normally play by myself, so when I knock down a gym I often do not put a Pokemon in it unless I know there are other players from my team in the area that are likely to jump in - then I'll put in a Pokemon and prestige it a little bit to make room for others. And as others mentioned - if you are the sole player in a gym that you just took down, it can make you a target for people who take such things a tad too seriously and go out and try to kick you out of your other local gyms. Just not seeing the crime against humanity, here, sorry, lol.
I wouldn't go as far as calling people idiots because they do not put any Pokemons in the gym. There's 2 reasons why I defeat a gym and do not bother putting anything in..
1. I already have a Pokemon in a gym and thus I will not get any stardust.
2. With all of the spoofers the gym will be taken down within 30min and put back to a level 10 gym.
So yeah, don't knock someone for not putting anything in. There's multiples ways of playing the game
I've kicked some valors out of a gym because based on their nicknames I suspect they are assholes, and left it grey so they wouldn't know who their enemy is. I've also seen them in a few consistent towers in the nearby park, they're hogging it big time. Can't take them all down 'cos that really wastes my time and battery, especially with all the lags and glitches.