Garen's burst isn't instant, not even close; all his silence does is keep you from escaping a few ticks of his spin (usually half of it). Bear in mind Garen's silence is also attached to an auto attack, which requires him to be in melee range of you to inflict it.
Talon could kill you before his silence even ended in some cases (albeit this wasn't often), whereas Garen won't do that unless its lategame, and you're the villain.
LeBlanc and Kassadin lost their silences for entirely different reasons (specifically because they were able to silence you, inflict their damage, and then escape before their silence ended so you couldn't trade back with them.
Edit: And before you call me a Garen player, I wouldn't be caught dead playing a filthy demacian.
for life.