You should list it, but don't expect it to hold much weight. When I am hiring post-docs, I count both the total number of publications and the total number of first authorship publications. This is what I use as my metrics for "publication strength", in addition, to some degree, to the impact factors of the journals. In neither of these metrics do I include manuscripts in preparation (although I do usually count submitted or manuscripts under review). However I do usually make an note of the number that the person lists as "under preparation". If the person makes it far enough to interview, I will usually then ask them what this actually means. (I have had a "manuscript in preparation" with my PhD advisor for almost 10 years now... so I could mean very little. His fault of course. The manuscript was prepared 9 years ago. He just hasn't looked at it yet.
If a large portion of your manuscripts in publication are "in preparation", you should consider addressing this in your cover letter. Speaking of which, have a cover letter, and make sure someone else reads it. I also have found it very helpful when applicants include a separate "research summary" document (1 page). And, follow up a week after you submit your application materials. You'd be surprised how something so little can help you stand out, because most applicants don't do it and it shows you are really interested. I can tell you honestly that this has made the difference between inviting an applicant to interview and passing them over.
(One other reason I think they are a good idea to list, even if I don't really count them as publications, is that I do sometimes rely on publication titles to help me understand an applicant's research background.)