Actually, now that I think about it, my residency did have psychiatrists who came to the ED. It didn't really change our practice significantly. Really, I only want to know one answer from them: "admit/transfer to psych unit" or "safe for discharge." If they (or a suitable surrogate) is not available, then I make that decision quite easily.
Theoretically, an in-house psychiatrist might reduce the rate of holds, but I didn't necessarily see that.
On an another but related note: I wish I could just say to most "suicidal" patients: "You're not really going to kill yourself, so I'll get your discharge paperwork and please follow up with a psychiatrist." I'd reserve holds just for the people I really think are gonna pull the trigger or at high risk for that.
Right now, I have to place a hold on anyone who says the magic words, even if they just want a sandwich.