By adults only do you mean explicitly violent or graphically sexual? Or are you just talking about the rating itself?
Personally, there's a special place in my heart for the existence of adults only games, because one of the things that makes video games such a special medium for me is that, for such a long time, up until the last couple of years when people started getting up in arms about them, video games were able to sneak under the radar of censorship and political correctness. Take games like Postal, for example. You can't get much more offensive than that, but I love the fact that it exists, because it's all about freedom of expression. Not just for the designers, but for people who play video games.
It's actually getting annoying now how games have to constantly be on the lookout for things people might find offensive, more so even than TV shows or movies. I think that sort of censorship actually stops games from taking risks and exploring new ideas, and that it's more likely to make games fall into the same cliches that 90% of TV shows or movies do. I respect games that don't hold back - it brings back that sense of video games as being an underground, rebellious medium not affected by society's rules.