Plenty of services can deal with X hours of downtime when a single machine fails for values of X that are longer than it takes to restore to a new machine from backups.
I'd like to add to this and say that a server being down for 6 hours, that if over the life of its uptime (months? years?) saves uncountable number of hours on computations and complexity, is so worth it.
Heck, even a machine like that being down for a week is usually still worth it.
I do not agree, and it is not my experience.Mind you that I've always worked in small/mid-sized businesses (50-300 employees) and basically every service has someone needing it for their daily work. Sure, they may live without it for some times, but you will make their lives more miserable.
And anyway if you already have all in place to completely rebuild every SPOF machine from scratch in a few hours, go the extra mile and make it an active/passive cluster, even manually switched, and make the downtime a minutes thing.
A small amount of work over a long period of time (i.e. setting up a redundant system) may be worse than losing a large amount of work in a short period of time.
Single machines just don't fail that often. I managed a database server for an internal tool and the machine failed once in about 10 years. It was commodity hardware, so I just restored the backups to a spare workstation and it was back up in less than 2 hours. 15 people used this service and they could get some work done, without it, so there was less than 30 person-hours of productivity lost. If I spent 30 hours getting failover &c. working for this system over a 10 year period, it would have been more hours lost for the company than the failure caused.
Heck, even a machine like that being down for a week is usually still worth it.