Yeah, I mean, when you do that, you're effectively turning away everyone who isn't logged in to DW (including all robots, as the OP is worried about.) Yes, people can click through the adult-flag, but a lot of people won't, especially if, say, somebody linked to it without mentioning whether it was work-safe or not, or if they have no real investment in finding out what's behind the warning.

Some journals need it, but the vast majority of people I know who use it, they'll post NC-17 fanfic about once every three months, and the rest of the time they're talking about their cats. IMO it really isn't something you should set "just in case".