Reviews and readers will expect these databases to be searched in most systematic literature reviews. You'll need a reason not to search them.
These database can be an effective complement to your search. They can be essential in their specialized topic areas.
Truely unbiased searches look for unpublished literature in a number of places, included meeting abstracts, white papers, clinical trial registries, and searching by hand.
Identifying sources for grey literature and being sure you've done enough is a challenge. The Canadian Agency for Drugs and Technologies in Health (CADTH) feels you pain and have produced a checklist that might help guide your grey research. The Grey Matters checklist provides an organized source of health technology assessment sites, regulatory agencies, trial registries, and other databases in a form that can help ensure the completeness of you search.
More information about trial registries and solving the problems associated with searching them is available through this site:
Medical and health-related trials registers and research registers
which is maintained by Julie Glanville and Carol Lefebvre and hosted by the York Health Economics Health Consortium.