I agree, it doesn't make it easier as such, although it's easier to race closely (taking the risk that whilst you may see them they may not see you), and does take a bit of getting used to, at first I was overreacting to any slight movement from the car next to me.
But the immersion is awesome, by far and away the best upgrade I think you can do, I recently 'had' to run one of the UKI oval races on my laptop away from home and the single screen just felt so wrong after running 3 for a while now.
Do It!
EDIT: There is a certain amount of being able to see the apex's, but I don't think it makes any significant difference, if you run single screen with lower FOV you just end up using other cues to hit the apex, if you're consistently not hitting the apex then maybe triples will work but it's not happening because of single screen if that makes sense...
But the immersion is awesome, by far and away the best upgrade I think you can do, I recently 'had' to run one of the UKI oval races on my laptop away from home and the single screen just felt so wrong after running 3 for a while now.
Do It!

EDIT: There is a certain amount of being able to see the apex's, but I don't think it makes any significant difference, if you run single screen with lower FOV you just end up using other cues to hit the apex, if you're consistently not hitting the apex then maybe triples will work but it's not happening because of single screen if that makes sense...