Do agents actually need memory, or are we using it to compensate for bad architecture?

I keep seeing memory treated as almost a default part of building an agent, and I'm starting to wonder if we're putting too many different things under the same label. Conversation history, user preferences, task state, retrieved knowledge, execution history. All of these are like pretty different problems, but they often end up getting handled through some kind of “agent memory” layer. That can create problems of its own. Stale information, conflicting state, bigger prompts, and a much harder time figuring out why an agent used a particular piece of information. I'm not saying agents shouldn't have memory. I'm more interested in what actually needs to persist for a system to work well. For those building agents in production, what do you actually persist, and what do you deliberately leave out?