The things is, people frequently complain about overpowered spellcasters, and this spell is a serious thorn in their eye. Any cleric supporting the enemy with a readied silence seriously threatens the viability of all spellcasters, requiring them to have and use the Silent spell feat (or a rod for it), or some items from the MIC (but which require swift activation, making it harder to use quickened spells), or use smart tactics depending on the terrain to be out of reach of the silence effect (like, around a corner, or somehow hidden).
You know, maybe, it's not a problem, maybe, it's actually good!