Ceremony, cocktail hour ( but those could be just done with just piano, no arranger needed ) and possibly during dinner although no one would care it the DJ just did backround music or there was someone actually playing a keyboard.
No matter how much preparation you put into a party like that no one going to accept someone behind a keyboard trying to sing every song. Not gonna happen. Even a good singer with tracks would be good for only certain spots here and there mixed with the DJ stuff. Its what people want and expect these days. Best I saw was a guy and girl who went out into the crowd with wireless mics and did some special "spotlight" songs. 95% of the night was DJ
Our venues as arranger players are very, very limited. I've worked in North Jersey, South Jersey and now South Carolina and I've never seen anyone else with an arranger. A few musicians I've met here don't even know what they are. Two keyboard players I've seen working here do the LH bass thing with drum machines and thats it.