It's really about making people pay for what things cost. That same person not traveling is still $1500/day but they get to go home at night and see their family.
When traveling they spend a lot of non-billable time shuffling around. Making them pay extra to get to do it makes it worse still.
The advantage of time + expenses is that the expenses vary based on the customer's location decisions. If you are in London they will be high. If you are Detroit they are low.