Look at Smith not declaring right now. Fear of a quickfire Sehwag innings.
It changes games, within ten overs, the field is pushed back, the bowlers start bowling tripe, fielders are defensive and every other batsman in the lineup benefits. You just have to watch Sehwag's effect on the rest of the lineup again, over the past few years.
A long innings will eventually demoralize bowlers, but it'll be a while and they can attack for a long time. What we see with really dominating innings is that even at the other end, bowlers start bowling tripe as they're off their rhythm, the fields all messed up, and the game seems to be taken away....
As I said, if I had two batsmen of similar averages I would pick the faster S/R 10/10 times. If the difference in S/R is 20+, let alone 30-40+, I'd probably pick the faster S/R even if the averages are 5-7 points lower.