Operation counters (insert, update, delete, etc.) are 32-bit. This means they wrap relatively frequently - in less than an hour at heavy load - making it difficult (if you have a complete timeseries at intervals shorter than the wrap time) or impossible (if you have only a single serverStatus) to get averages over periods longer than that. There are diagnostic situations where it would be useful to answer questions like "has the mongod instance seen a large number of deletes since it was started"; if we used 64-bit counters we could answer such questions. Performance impact would need to be assessed, but I suspect it won't make a measurable difference.