And then there is the umpire who tells folks, "yeah, but I pace myself".
IOW, nobody gets 100% because you need to last longer than you should be out there.
Don't get me wrong. I am aware of special circumstances where it may be unavoidable. I must note that in my area when we have an umpire go down for any reason, the rest of the umpires step right up, often without being asked, and take on additional games with minimal breaks.
However, as a routine so an assignor doesn't have to deal with more people than necessary or because an umpire may be greedy or an association has overbooked their weekend just isn't right. None are an acceptable reason to place the umpires' integrity and health, or the association's reputation in jeopardy.