Golmaale,

there have been several court decisions in Germany about not using single coupons for flights - all so far in favor of the passenger. There even have been lower court decision that it is ok to use the inbound but not the outbound sector - but no higher court yet.
It seems that the airline try to avoid court cases. They likely don't want to get a higher court deciding against them.
Similar situation in other countries. I happen to have a great travel agent (he is worth more than the fees he charges) who can get me RTWs even starting in Vietnam and other low cost places). So i am buying there.

The only problem i can see is when you check luggage and don't want to have it checked to the final destination. You should come up with some story (like you meet somebody at the airport and want to give him/her a bottle of wine in your checked luggage.....).
You then can feel unwell at your real destination and ask to offload you to avoid beeing a no show and have the flight waiting for you / paging you etc.