What is the disadvantage of an excursion fare?

Travel agent: Good morning. Can I help you sir?
Mr. Adams: Yes, good morning. Er, my name is Adams. I am going to a conference in Sydney in July.
Travel agent: Yes. How long for?
Mr. Adams: I expect to be there for about three weeks.
Travel agent: I see. Well, there is an excursion fare.
Mr. Adams: How much would that be?
Travel agent: 795 pounds. That allows you one stopover, one stopover outside Australia.
Mr. Adams: Only one stopover?
Travel agent: Yes, (7[B])that’s a restriction on excursion fares, one stopover and not in Australia.
Mr. Adams: I see. Well, I suppose it would be nice to have a stopover in the far east somewhere, Hong Kong or Bangkok. Hm. And there’s another point. Er, I’m going with two friends and we have got a friend in Perth in Australia. Now we all know him and it would be very nice to visit him on the way back to London after the conference.
Travel agent: Sorry sir. (8[C])You’ve got to remember, no stopover in Australia on the excursion fare. Of course, you could pay the full fare.
Mr. Adams: Mm. No, I don’t really want that.
Travel agent: Or, well, there is something else you can do. You could take a single ticket to Sydney, that’s 400 pounds, (9[B])and a ticket from Sydney to Perth, about 150 pounds, and then another single Perth to London ,that’s about four hundred and twenty. That’s much less than the full fare. But remember, no stopovers allowed on single flights, so you wouldn’t be able to go to Bangkok or Hong Kong.
Mr. Adams: I see. Well, I’d just fly directly back to London. But look, if you don’t mind I think (10[D])I’ll talk it over with my friends first and I’ll come back in a few days’ time. Urn, I hope they’ll come to Perth with me. I’d really like to see my friends. Anyway, um, I’ll see you later.
Travel agent: That’s fine. I’ll see you next week perhaps. Goodbye.
Mr. Adams: Bye.