I know there are some other threads on this, but most are rather old and the only newer one is a guide on how to add it yourself, but it should really be an option from in-game.
First, for those who don't know what it is (since half the debates on the issue arise from people not understanding what it is). Smartcasting is a system in LoL where instead of hitting a targeted ability and then click on where you use it, it instead cast to where your mouse cursor currently is (bypasses the click needed to activate the ability). An example: let's say you're playing windrunner, normally to powershot you need to hit w (by default) and then click where you want to aim, with smartcasting, you would hit w and you would start aiming toward where your cursor was when you hit the button.
Smartcasting does NOT aim for you, prioritize your spells, or any other such nonsense, it merely removes the mouse click when using abilities.
This is a great convienence option, especially for those used to it from LoL. I've seen come talk about how it decreases the skill cap or something, but from LoL experience I can tell you it does the opposite (why that micro second faster makes a difference for some people, I don't know, for some it has the reverse effect). If anything smartcasting allows many people to fight the controls less and their enemies more.
I've had many a friend ask me about smartcasting in LoL as it really is a nice feature that just makes the game flow smoother for some people (plus, if you don't like it, don't use it, if you time your moves right the difference is negligible to non-existent).
For smartcasting options, it would be great to have both a toggle to make all skills smartcast (and possibly items, though that would be best if a seperate option), along with the option to bind them individually by ability (not needed, but is nice for those who like certain skills smartcasted and others not).