According to Answers in Genesis' list of arguments not to use (http://www.answersingenesis.org/home/ar … nt_use.asp)

Argument not to use # 9: There are no beneficial mutations

"This is not true, since some changes do confer an advantage in some situations. Rather, we should say, 'We have yet to find a mutation that increases genetic information, even in those rare instances where the mutation confers an advantage.'”

First, AiG tells its readers that there are some beneficial mutations, but the problem is what comes after: "We have yet to find a mutation that increases genetic information"

In an earlier question, Neil Gostling told me that tandem duplication is a mutation that would explain the increase of genetic information.

Why is AiG telling its readers that there is no mutation that increases genetic information?

Hmm, I think that's not a biology question but a sociology and psychology question, and we might not want to turn this site into yet another creation-evolution "debate" site, so I'll keep it brief here.

For more refutations of this blatantly false assertion see for example:
http://www.talkorigins.org/indexcc/CB/CB102.html

Possible reasons (I'd say all of them likely to be true to some degree, less so #1):
1. They don't know or understand gene duplication yet
2. They know, as people have continually pointed this out, but prefer to maintain the misinformation until more people know and they are forced to add this to their "arguments not to use" list
3. They don't care about providing accuracy on scientific information; proselytizing is more important, or even spreading misinformation
4. They believe they have found evidence to the contrary (albeit evidence that science has not supported)