Two towers is a big advantage. That gives other lanes an option to come mid or other lane and push harder.
If your jg is clearing out instead of trying to save the towers then he is in a trouble. How do you think enemy will switch lane ? by walking thru' jungle and WARDING as they go.
Once warded they'll single him out and get that kill. Towers give you a security in your jungle. All your logic makes sense ONLY if enemy is as stupid as you sound them to be.
Basics of winning a game.
- Objectives
- Drag/Rift/Baron
- Warding
If a devour jg does just farming while his team is losing, he might not get help once he has items. Losing lanes means teammate won't come to his rescue and hell they might even FF at 20.