If that were true, just kill the rams and cats and voilà their hammer is useless.
But it isn't. A teuton hammer with 75k clubs won't even come close to having none at home ... don't forget, that there's a 1k outgoing attack limit, ergo max 2k raids on their way. You usually don't have an average of 38 clubs per farm. Also, farming with TK isn't the best way to go (speed low, cost high when spiked), so you can kill at least them. Did you even ever try to kill a hammer at home or are you just assuming stuff without any knowledge and experience?
No, you are right, it's best to leave troops at home whenever you're not at pc. What are you even saying?
When I know that I won't be available for X hours, then I'm sending away whatever I have left. As I assume every decent player.
And I'm talking from perspective of having a hammer, but if you wanna kill the hammer of some inactive then sure, everything works.