The current travel requisition/expense report works in the following way:
There is a Expense Management parameter requires travel to be pre-authorized before claiming the expense. The flag is "Pre-authorization of travel is required". With this switched on, Expense reports must have a travel requisition mapped to each line before submitting for approval.
When this setting is on, the system behavior is as follows:
Raise travel requisition - Flights, $1000. Approved travel requisition
Raise expense report. map to approved travel requisition.
Create line with Flights, $800.
Map line to travel requisition
Submit Expense report

Firstly, there is a bug in the functionality at present which means the pre-authorization can be switched off. I have logged this.

Secondly, the way in which the functionality works means the following:
All travel must be pre-authorized or none at all
If the "pre-authorization required" flag this is not switched on, then the Expense report lines must be separately approved even through a travel requisition has been mapped to the expense line. There is nothing in the Expense report workflows to detect whether a travel requisition was mapped to an expense line. This is the biggest gap in functionality. This should work in a similar way to the invoice matching to purchase order workflow i.e. detect if there is a match and allow automatic approval.

The functionality should be expanded. Some ideas are:
Require pre-authorization on some categories but not others. Most organisation require pre-authorization of flights, accommodation and entertainment but not other minor categories such as taxis, etc.
Provide attributes in the Expense report workflow which detect whether a travel requisition has been mapped to an expense line. This would allow automatic approval for lines with a travel requisition. To me, this is a simple change which covers almost all scenarios.