If creationism acted only as a watchdog on scientific research then their participation would be much more welcome, but they instead focus most of their effort on the dissemination of scientific misinformation in an effort to convince laypeople that creationism is a legitimate scientific alternative to evolution.
That being said, I think Creationists serve a somewhat positive role in a narrow niche by making popular representations of evolution more accurate. For example, they helped correct the mispresentations contained for a while in biology textbooks about Haeckel's drawings and Kettlewell's moths. But their efforts along these lines are primarily detrimental to science, since their true goal is to replace treatments of evolution in public schools with their own religious views.
Creationists have virtually no voice in scientific circles. They do not contribute to mainstream scientific conferences or journals. Scientists only engage creationists outside scientific settings, and then only to counter general misinformation, for instance, in a debate or a layman level book.
--Percy
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