Many people have stated they have had more success getting offers accepted when working directly with the listing agent (and no agent of their own). One reason could be that information flows faster when there are fewer people through which it must be relayed.
Even the most dedicated agent cannot be instantly available 24/7 - and is not expected to be. They have other clients and their own personal lives. If the buyer's agent is working with another client, a counteroffer or request for more documents may not get relayed to the buyer immediately. That delay may hinder back-and-forth negotiations and subtle information may get lost. For example, perhaps a seller cares mostly about the price, or perhaps the seller cares more about other terms like number of days until closing, number of inspection days, or other contingencies.
The situation where each side has their own agent reminds me of the children's game "telephone", where a message is relayed through many people and much of it ends up getting garbled.
In some situations, it would seem only having one agent involved - working as a transaction broker - would make it easier for each party to figure out what terms are most important to the other, so that each side gets as much of what they want as possible. Where there are multiple offers, the buyers working with the listing agent would seem to be in the best position for negotiations.