I'm hoping that the Wedding venue Lot type is associated with having NO random sims visit the lot and very minimal autonomous activities outside of what the events prompt, so that guests pay attention to the things that the player is controlling for the events, rather than "doing other stuff".
@Daephene, if you are open to using mods, LittleMsSams has a mod that lets you get bartenders (and DJ's, and many other service roles) on lots with lot types that do not normally get them. It is a fantastic mod, as it means you can put anything which requires a 'service sim' on any lot type and get the roles auto-filled. Even works to get vendors for all the various food (and other vendor) stands.