There is no benefit to increasing your sight range past the mini map size, except to clear more fog of war. Using the shift key to move the screen focus lets you hit any enemy you can see with magic. However, enemies not shown on the mini map are not displayed when you move the focus over that area, so basically the range is a couple of screen heights/widths away from your character (I was using 1024x768 resolution).
With my warrior, I kept my sight range at about the screen size, to help keep track of where I had explored. I put one point into Elven Sight at the start, then eventually started avoiding equipment with sight bonuses.
Elven Sight is much better than Ranger Sight.
Elven Sight (Passive)
You can see further and thus target and attack your enemies from afar.
Level 1 Increase sight range by 2. Required level 1
Level 2 Increase sight range by 4. Required level 5
Level 3 Increase sight range by 8. Required level 9
Level 4 Increase sight range by 10. Required level 13
Level 5 Increase sight range by 12. Required level 17
Ranger Sight (Passive)
Grants you the keen eyesight of a ranger to help you see your enemies from afar.
Level 1 Increase sight range by 1. Required level is 6
Level 2 Increase sight range by 1. Required level is 12
Level 3 Increase sight range by 1. Required level is 18
Level 4 Increase sight range by 1. Required level is 24
Level 5 Increase sight range by 1. Required level is 30
