Pain train cap bonus is not so much about last caps / backcaps, but more on quick transitions.
Say you are pushing spire and chasing the med/frags at the same time. Having the demo capping allows better classes to go for the chase. The same can work after a midfight win.
Sometimes scouts can't reach the cap fast enough (for exemple pushing out of blands last).
In these common cases pain train gives an advantage, and yes this 10% damage makes a difference (don't ever run it to granary mid).