Audiences these days can’t be trusted only to applaud the good stuff: give them half a chance and they’ll cheer the downright mediocre as well. And there’s no doubt that too much applause in the middle of a symphony, opera or concerto can definitely break up its drive and flow.
Besides, if people get the idea that it’s fine to clap at the end of movements, they’ll start clapping at every false ending as well, with disastrous consequences. I’m having visions here of people bursting out enthusiastically half a dozen times within ten minutes at the end of a Haydn symphony: something I’m sure no one thinks is a very good idea.
If asked, then, I’ll continue to tell novices to classical music that, in general, one doesn’t applaud until the end of the whole piece