I like this idea, though I feel like it would be rather awkward in terms of gameplay. I understand it would of course be a nuisance to casters that wouldn't remove their kit for several seconds, however knowing why I'm suddenly incapable of casting spells for an extended amount of time feels a lot more natural than just all of a sudden doing a combo wrong - ie; Pressing buttons in rapid succession with the silence at the start or middle of casts.
I think the reason that Riot decided to keep this form of micro-crowd control is to do just that, stop momentum without ruining the chance to escape a chase or be out of a team fight for extended amounts of time. Silencing I think is that middle ground where it doesn't prohibit a champion from escaping or positioning themselves to still help the team once the effect has worn off. If we just did a micro-silence it would feel aggravating because there would be no time to properly recognize that it's happened which results in either the user to feel like they're not being relevant or the target feeling angry that they just did their combo wrong with no way to fix it later.
I would propose something like a circumstantial silence. By this I mean, "Target can cast one spell before being silenced for X seconds. This effect wears off after X seconds." It would require a player to think about whether or not they want to REALLY cast that spell and risk being silenced or if it would be better to just wait for the effect to wear off and get a full combo off to help their team.
For example; A soraka wouldn't have any problem with deciding to use her ult if her team needs it, and the silence would only remove her relevance for a short amount of time before she could start to support again. But if we look at a Ziggs player in a similar situation, he could use Q to get fast and easy damage, W to try and bump himself or enemy champions a certain way, E to try and get an AOE slow, or his Ult to try and get some fast and easy damage similar to his Q with the risk of it missing entirely and having to wait for it's cooldown and without the rest of his kit.