Certainly presenting the offer "in person" is the optimal way to present an offer. That way you can get your buyer's entire message across the Mr. & Mrs. Seller, and make sure they heard all the pertinent information, and have any questions they might have about your offer answered.
Unfortunately, as Mary Starkey points out, in today's fast-paced world, in-person presentations of offers are becoming more and more rare. Often, the seller does not want to be bothered by the buyer's agent. They don't want to be "sold" the contract, rather they'd like to sit with their own agent and discuss the offer in private.
Sometimes it's simply because of scheduling. With a purchase offer, time is of the essence, and often the fastest way to schedule an offer presentation is via phone, sometimes a conference call. Often the listing agent can't even manage a face-to-face with their client, and they'll present the offer via telephone, occasionally with one partner at work, and the other at home.
I, too, have found that more and more of my offers are presented via telephone, rather than in person, and rarely is the buyer's agent present. The optimal system?... no. But expedient, and functional.