From what I have felt playing Varus is that he is a champion all about choices. Every ability and his kit make you make choices. His passive can make you decide whether you want to CS before hitting champs or just attack. His Q, although a bit clunky, makes you decide between damage and efficiency. Using E and R at the right times can also make a big difference. At the beginning of a fight they can lend a lot of CC that is necessary. However, waiting for a better cluster of enemies can sometimes be better if the enemy does not have a strong engage.
And then there is the W procs. First, W is a great help in the laning phase in last hitting. That extra AA damage is noticeable. The way I usually play Varus is that the W procs are perfect for small skirmishes and buff control. In the mid game the stack procs can help secure kills in small engagements. When taking baron and Dragon, sometimes it is better to proc the stacks in order to get chunks of health down to throw off smite steals.However, as the game transitions into late game, I find the procs are more for extra damage rather than relying on the procs. Yes, you can focus down a single target by focusing on procs (have done this numerous times to remove a fed Mundo from a fight) or you can play as most ADC in late game and focus on the auto attacks. If you don't need to proc the stacks to ensure a vital kill then sometimes it is worth it to save your skills for the AOE.
These decisions are part of the reason why I love Varus. He relies more upon in the moment decisions then other ADCs that I play (Kog, Ashe, Sivir). Even though his kit can feel kind of clunky through micromanaging the blight stacks I find that it does become easier over time.