Air carrier Captain reported refusing an aircraft that had inoperative autothrottle's.

Narrative

Bottom Line: Refused aircraft for inoperative autothrottle system. Narrative: We were given an aircraft with an inoperative autothrottle system for a scheduled ZZZ-ZZZ1 turn. We would have the aircraft for both legs; so scheduled for nearly 6 hours of flying when combined. In addition; this was day four of a four day trip for us and legs two and three of the day that would push us close to our maximum duty day (we had already worked leg one from ZZZ2-ZZZ prior to this). I felt that under the circumstances; the safest course of action was to refuse the aircraft. This is an MEL that on many days I would accept. That said; I believe that total flight time with this MEL combined with a long day at the end of a relatively long trip was not smart. Upon learning that we were assigned this aircraft; I immediately called the Dispatcher working the flight to discuss and tell him I did not feel comfortable. This was still approximately an hour from departure; so my hope was to minimize operational and customer disruption with a tail swap prior to boarding. The Dispatcher was able to talk with the coordinator and immediately got us a new aircraft. We took a minimal delay for this (we were already delayed anyway) and received a suitable aircraft for our ZZZ1 turn. Cause: I think the only way to fully mitigate the risk is to fix relatively serious MELs such as this one. Possibly when the aircraft is in a company maintenance base RON (Routine Overnight). I'm sure Maintenance is swamped right now as we're in the middle of summer flying; so easier said than done and I'm sure their are higher priority MELs to fix first. As stated above; under different circumstances I would accept this MEL if I felt I could safely execute the flight.