One need only look at the revenue stream of agencies to quickly realize that most income is generated from commissions. I don't know of any trade show producers that pay a commission on booth sales. Because agencies don't see trade shows as a revenue source, they are reluctant to recommend them when assisting clients with marketing plans. I have seen companies attend shows with beautifully designed, expensive literature to be used in a drab, unappealing exhibit that wouldn't attract visitors if you were giving away gold. In other cases, companies have come to shows with literature that was intended to be used as a direct mail piece or an annual report - not as a trade show piece.
Different jobs require different tools.
Trade show selling is different. It requires a different approach to the potential client. Agencies still have invaluable creative resources, a keen sense of what attracts attention and copy writing skills that most of us lack. Make sure your agency knows why you are going to exhibit, what your objectives are, and what contribution you expect from them. In many cases you will know more than the agency.