Is it an obligation? As in, legally you have to buy them or else, you could lose the trademark in court for not protecting it?
Otherwise, I'd say no one is forced to own all possible TLD variations of their domain name, get the .org, .net, at most, don't worry about the others. As more TLDs are created, it would be foolish to chase them all and this mentality may soon be relegated to old 2000 marketing books.
It reminds me of the people in the 90s who were building dozens of duplicate web sites all under a slightly different keywords related to their business. Nobody does that anymore.