Second, most of these consultants don't know me or my firm. All firms are not the same. The culture, habits and processes of the attorneys in my firm are unique and a "one size fits all" approach won't work for us. The fact that they listen to my feedback puts me in a unique position to influence the developoment of the product to meet our particular needs.
Third, I'm starting to hear from vendors that they will not deal directly with these firms. So it seems to me likely that just using them as advisors would stand a good chance of wiping out any savings you may realize from their services. When you couple this with the Non-Disclosure Agreements most of us are forced to sign with our vendors, it makes this situation problematic.
This is not meant to condemn these folks or deny them an opportunity to earn a living as they choose. Some of these consultants, having worked for a particular vendor in the past, can provide unique insight into that vendor's sales practices and processes. Much like the former IRS agents who open tax consultancies. Some Librarians don't like to negotiate and this is an alternative for them to avoid the "unpleasantness" of the negotiating process. My view is that these consultants may have some value as advisors. However, they can interfere with the partnership between the firm and the vendor.