To Vendor or Not to Vendor.... That is the Question

Should we allow vendors (salespeople) to be members of our group and attend meetings?

Pro:  Since our group is fairly small and we rely on vendors for many of our presentation the advantage is that we build better relationships and our membership gets larger.  We  also have access to more information and vendors may have insight and/or perspectives we hadn't considered.

Con:  Since some of our questions to each other are about vendors, it might infringe on "free discussion" about those vendors.  No one wants to bad mouth a company while they're in the room.

I'd be interested in your thoughts.