From the Wiki
*Rat-Ta-Tat-Tat is a self-buff ability that also causes Twitch's basic attacks to be replaced by pass-through linear skill shots. *
Usually Twitch is using this where there is a crowd of enemies in a confined area - such as in the jungle or caught in a lane, or by an ally's CC. This lets him shoot with reckless abandon. And even though it is possible to sidestep them, the projectiles are pretty darn quick so its still pretty reliable.
However, since you do have to initiate an Auto Attack, which then shoots a Skill Shot, it means that you can't use it in the same way as a regular Skill Shot. As you mentioned, you can't "lead" the target. But you also can't blindly fire into the Fog of War like you could with a regular Skill Shot.
This would be a simple fix, though. You could pretty easily code it to disable Twitch's Auto Attack during the duration, and instead let him re-cast "R" to fire a skill shot normally, with the Cooldown equal to his Attack Speed. This would let Twitch just shoot it where ever he wanted without needing an actual target.