further to Dtwclipper's comments,
I am working with an IT
company providing solutions to the airlines... a major problem is that the airlines cannot agree on WHAT information is to be exchanged, who maintains the E-TKT database, what happens to the E-TKT status, when a flight is cancelled after the passenger has checked-in, what if the a/c has to return to the departure city, etc etc. The list is pretty long. IATA has only set a few standards.
Basically, airlines hosted on the same reservation system, there should not be any problem. Linking the various system to allow interlining on an e-tkt basis is very very costly.
Then, the airlines also face legal restrictions, i.e. the government REQUIRES a paper ticket before you can even enter the check-in area.
Or, if the departure control system (check-in system), is NOT linked to the reservation system, the data often cannot be exchanged.
the US airlines are leading here, AA
for example has a rate of some 95% on E-tickets, but this of course due to the high domestic travel. as soon as you want to code-share...
This is just an eagle's eye view, it's very complex.